93 C# interview questions to hire top developers

Evaluating C# developers requires a strong understanding of the language and its nuances. As technology evolves, it is helpful to have a go-to set of questions, ensuring you’re well-prepared to assess candidates at every level, similar to how we have discussed skills for a software developer.

This blog post provides a question bank categorized by experience level, starting from basic to expert C# questions and even some multiple-choice questions (MCQs). Use these questions to help you gauge a candidate's knowledge of C# concepts, coding abilities, and problem-solving skills.

By using this comprehensive list, you can confidently assess candidates and make informed hiring decisions. To further streamline your screening, consider using Adaface's C# online tests to quickly identify top talent before the interview stage.

Table of contents

Basic C# interview questions

1. What is C# and why do we use it?

C# is a modern, object-oriented, type-safe programming language developed by Microsoft. It's designed for building a wide range of applications that run on the .NET platform.

We use C# because:

• It's versatile: Suitable for web, desktop, mobile, game (Unity), and cloud development.

• It offers robust tooling: Visual Studio provides excellent IDE support.

• It has a large community: Abundant resources and support are available.

• It provides automatic memory management: Through garbage collection.

• It offers features like LINQ for querying data, async/await for asynchronous programming, and generics for type safety. E.g.

  // Example of a simple C# program
  using System;
  
  public class HelloWorld
  {
      public static void Main(string[] args)
      {
          Console.WriteLine("Hello, World!");
      }
  }
  

2. Explain the difference between value types and reference types in C#.

Value types and reference types differ in how they store data. Value types (like int, bool, struct, enum) directly hold their value within their allocated memory. When you assign a value type variable to another, a copy of the value is created. Each variable then has its own independent copy.

Reference types (like string, class, array, delegate) store a reference (a memory address) to the actual data, which is located on the heap. When you assign a reference type variable, you're copying the reference, not the data itself. Both variables will then point to the same memory location. Modifying the data through one variable will affect the other.

3. What is the purpose of the 'using' statement?

The using statement in C# (and similar constructs in other languages like Python's with statement) is used to ensure that resources are properly disposed of, even if exceptions occur. It provides a convenient way to automatically call the Dispose() method on an object when it goes out of scope.

Specifically, the using statement works with objects that implement the IDisposable interface. It automatically generates a try...finally block. The object is created and used within the try block, and the Dispose() method is called in the finally block, guaranteeing resource cleanup regardless of whether the code within the try block completes successfully or throws an exception. For example:

using (Font font3 = new Font("Arial", 10.0f))
{
    // Use font3
}
// font3.Dispose() is automatically called here

4. Describe the concept of inheritance in C#.

Inheritance in C# is a mechanism where a new class (derived or child class) acquires properties and methods of an existing class (base or parent class). This promotes code reusability and establishes a hierarchical relationship between classes. The derived class can inherit members (fields, properties, methods, events, etc.) from the base class and can also add new members or override the inherited ones, thus extending or specializing the base class's behavior.

Key aspects of inheritance include:

• Code Reusability: Avoids redundant code by inheriting from existing classes.
• : Syntax: Used to specify inheritance (e.g., class Dog : Animal).
• virtual and override: Allows derived classes to modify the behavior of inherited methods. The base class method needs to be declared virtual, and the derived class uses the override keyword.
• base Keyword: Used to call the base class's constructor or method from the derived class.

5. What are interfaces in C# and how are they different from abstract classes?

Interfaces in C# define a contract that classes can implement. They contain only declarations of methods, properties, events, and indexers, but no implementation. A class can implement multiple interfaces.

Abstract classes, on the other hand, can contain both declarations and implementations. They can have fields, constructors, and methods with bodies. A class can inherit from only one abstract class. The key difference is that interfaces enforce a 'can-do' relationship, while abstract classes establish an 'is-a' relationship. Also, interfaces support multiple inheritance while abstract classes do not. Example:

interface IMyInterface { void MyMethod(); }
abstract class MyAbstractClass { public abstract void MyMethod(); }

6. What is polymorphism and how is it achieved in C#?

Polymorphism, meaning "many forms", is the ability of a class or interface to take on many forms. It allows objects of different classes to be treated as objects of a common type. In C#, polymorphism is achieved through several mechanisms:

• Inheritance: Derived classes inherit from a base class and can override virtual methods. This is runtime polymorphism.
• Interfaces: Classes can implement interfaces, providing different implementations for the same interface methods. This also facilitates runtime polymorphism.
• Method Overloading: Creating multiple methods with the same name but different parameters within a class. This is compile-time polymorphism.
• Abstract Classes: Abstract classes define abstract methods that must be implemented by derived classes, enforcing a certain structure and polymorphism.

For example:

public class Animal {
 public virtual string MakeSound() { return "Generic animal sound"; }
}

public class Dog : Animal {
 public override string MakeSound() { return "Woof!"; }
}

public class Cat : Animal {
 public override string MakeSound() { return "Meow!"; }
}

Animal myAnimal = new Dog();
Console.WriteLine(myAnimal.MakeSound()); // Output: Woof!

7. Explain the difference between '==' and '.Equals()' in C#.

In C#, == and .Equals() are used for comparing values, but they differ in their behavior, especially when dealing with reference types.

• == : This is the equality operator. By default, for reference types, it checks for reference equality meaning it verifies if two variables point to the same memory location (i.e., are they the same object instance). For value types, == compares the values of the operands. However, the == operator can be overloaded by classes and structs to provide custom equality logic.
• .Equals() : This is a method inherited from the System.Object class. By default, for reference types, its behavior is similar to == checking for reference equality. However, its main purpose is to be overridden in derived classes to provide value equality. Value equality means that it checks if the contents of two objects are the same based on defined criteria within the class. Value types typically override the .Equals() method to compare values and not memory addresses.

8. What is a delegate in C#?

In C#, a delegate is a type that represents references to methods with a particular parameter list and return type. Essentially, it's a type-safe function pointer. Delegates enable you to treat methods as objects, allowing them to be passed as arguments to other methods, stored in data structures, and invoked dynamically.

Think of it as a placeholder. You declare a delegate type, then you can create instances of that delegate that 'point' to specific methods. These methods must match the signature (parameter types and return type) defined by the delegate. Delegates are fundamental for implementing events and callback mechanisms in C#.

9. What are events in C# and how are they used?

In C#, events are a way for a class or object to notify other classes or objects when something of interest happens. They are based on the delegate type and provide a mechanism for loose coupling between objects. An event allows a class to expose notifications without exposing the underlying delegate directly.

Events are used through the following steps:

• Define a delegate: This defines the signature of the event handler method.
• Declare an event: Using the event keyword, based on the defined delegate.
• Raise the event: When the event occurs, the class raises the event, which invokes the registered event handlers.
• Subscribe/Unsubscribe to the event: Other classes or objects can subscribe to the event to be notified when it is raised using += and unsubscribe using -=.

For example:

public delegate void MyEventHandler(object sender, EventArgs e);

public class MyClass
{
 public event MyEventHandler MyEvent;

 protected virtual void OnMyEvent(EventArgs e)
 {
  MyEvent?.Invoke(this, e);
 }

 public void DoSomething()
 {
  // ... some code
  OnMyEvent(EventArgs.Empty); // Raise the event
 }
}

10. Describe the purpose of exception handling in C#.

Exception handling in C# is a mechanism to deal with runtime errors, ensuring that a program doesn't crash unexpectedly and can recover gracefully. It involves monitoring a block of code for potential errors, catching those errors when they occur, and then executing specific code to handle them, like logging the error or attempting a retry.

The core purpose of exception handling revolves around these key elements:

• Robustness: Prevent application termination due to unexpected issues.
• Error Reporting: Provide meaningful information about errors for debugging and logging.
• Resource Management: Ensure resources are properly released, even in error scenarios (e.g., using finally blocks).
• Code Clarity: Separate error-handling logic from normal program execution, improving readability.

11. What is LINQ and what are its benefits?

LINQ (Language Integrated Query) is a set of features introduced in .NET that provides a unified way to query data from various sources. These sources can include databases, XML documents, collections, and more. It allows you to use a consistent syntax to perform operations like filtering, sorting, grouping, and joining data, regardless of the underlying data source.

The benefits of LINQ are numerous:

• Readability and Maintainability: LINQ queries are often more concise and easier to understand than traditional SQL or code-based data manipulation.
• Type Safety: LINQ provides compile-time type checking, reducing runtime errors.
• IntelliSense Support: Visual Studio provides IntelliSense support for LINQ queries, making it easier to write correct code.
• Data Source Agnostic: The same LINQ syntax can be used to query different data sources.
• Productivity: LINQ simplifies data access and manipulation, leading to increased developer productivity. Example: var result = from x in list where x > 5 select x;

12. What is an extension method and how do you create one?

An extension method allows you to add new methods to existing types without modifying the original type's definition. This is useful when you want to add functionality to a type you don't control (e.g., a class from a third-party library or a built-in type like string).

To create an extension method in C#, you define a static method in a static class. The first parameter of the method specifies the type that the method extends, and it's preceded by the this keyword. For example:

public static class StringExtensions
{
    public static string ToTitleCase(this string str)
    {
        // Implementation to convert string to title case
        return System.Globalization.CultureInfo.CurrentCulture.TextInfo.ToTitleCase(str.ToLower());
    }
}

Now you can call ToTitleCase() as if it were an instance method of the string class:

string myString = "hello world";
string titleCaseString = myString.ToTitleCase(); // titleCaseString will be "Hello World"

13. Explain the difference between 'const' and 'readonly' keywords.

Both const and readonly are used to prevent modification of a variable after its initial assignment, but they operate at different levels.

const means the value of the variable is known at compile time and cannot be changed at runtime. It's a compile-time constant. On the other hand, readonly means the variable can only be assigned a value during the declaration or within the constructor of the class/struct it belongs to. Its value is not necessarily known at compile time and can be different for different instances of the class. Consider these examples:

public class Example {
 public readonly int runtimeValue;
 public const int compileTimeValue = 5;

 public Example(int value) {
 this.runtimeValue = value; // Allowed
 //compileTimeValue = 10; // Not allowed
 }
}

14. What are generics in C# and why are they useful?

Generics in C# allow you to define type-safe data structures and algorithms without committing to a specific data type. You use type parameters (e.g., T) as placeholders, which are then replaced with concrete types when the code is used. They avoid boxing/unboxing overhead and enable compile-time type checking, preventing runtime errors.

Generics are useful because they promote code reusability, type safety, and performance. Without generics, you might use object type, requiring casting and potentially introducing runtime errors. With generics, you can write code that works with different data types in a type-safe manner and improve code clarity. For example, List<int> is a list of integers, and List<string> is a list of strings, both using the same List<T> definition. This eliminates redundant code.

15. Describe the purpose of attributes in C#.

Attributes in C# provide a way to add declarative information to code elements (assemblies, modules, types, members, parameters, etc.). They are used to store metadata about these elements, which can then be accessed at runtime using reflection. This metadata can be used for various purposes such as:

• Serialization: Specifying how objects should be serialized. For example, the [Serializable] attribute marks a class as serializable.
• Code analysis: Providing information for static analysis tools.
• Compiler directives: Influencing compiler behavior. For example, [Obsolete] attribute marks code as deprecated.
• Runtime behavior: Modifying the behavior of code at runtime. For example, Attributes are used extensively in ASP.NET Core for routing and validation.

Attributes enhance code readability and maintainability by separating metadata from the core logic. They reduce the need for complex configuration files or code-based metadata management.

16. What is boxing and unboxing in C#?

Boxing is the process of converting a value type (like int, bool, struct) to a reference type (object). It implicitly creates an object on the heap and copies the value type's data into it. Unboxing is the reverse process; it explicitly extracts the value type from the object.

For example:

int i = 123;
object box = i;  // Boxing
int j = (int)box; // Unboxing

Boxing involves memory allocation and copying, so excessive boxing/unboxing can negatively impact performance. Unboxing also requires an explicit cast, and a InvalidCastException is thrown if the object doesn't contain the correct value type.

17. Explain the difference between 'as' and 'is' operators.

The is and as operators in C# (and similar languages) serve different purposes related to type checking and type conversion.

• is: This operator checks if an object is compatible with a given type. It returns a boolean value (true if the object is of that type or can be implicitly converted to that type, and false otherwise). It does not perform any type conversion. For example: if (obj is string) { // do something }
• as: This operator attempts to convert an object to a specified type. If the conversion is successful, it returns the object as that type. If the conversion is not possible (e.g., the object is not of that type or cannot be converted), it returns null. For example: string str = obj as string; if (str != null) { // use str }

18. What is the purpose of the 'sealed' keyword?

The sealed keyword in languages like C# and Java (less directly) is used to restrict inheritance. When applied to a class, it prevents other classes from inheriting from it, ensuring that the class's implementation remains final and cannot be modified or extended through inheritance.

This is useful for:

• Security: Preventing malicious derived classes from altering behavior.
• Performance: The compiler can optimize sealed classes better because it knows the exact type at compile time.
• Version Control: Changes to a sealed class won't unintentionally break derived classes in other assemblies.
• Design Control: Preventing unexpected extension.

19. Describe the difference between stack and heap memory.

Stack memory is used for static memory allocation, such as local variables and function call data. It's managed automatically by the system using a LIFO (Last-In, First-Out) structure. It's faster to access but has a limited size.

Heap memory, on the other hand, is used for dynamic memory allocation. Memory is allocated and deallocated explicitly by the programmer using functions like malloc()/free() in C or new/delete in C++. Heap access is slower but it provides more flexibility and a larger memory pool. Memory leaks can occur if heap memory isn't properly deallocated.

20. What are nullable types in C# and why are they useful?

Nullable types in C# allow you to assign null to value types (like int, bool, DateTime, etc.). Value types are, by default, non-nullable, meaning they must always have a value. Nullable types are declared using the ? symbol after the type, e.g., int?, bool?.

They're useful when you need to represent the absence of a value for a value type. For instance:

• When dealing with database fields that can be NULL.
• Representing optional data in data transfer objects (DTOs).
• Indicating that a value hasn't been initialized or is unknown.

int? nullableInt = null; 
if (nullableInt.HasValue)
{
 int value = nullableInt.Value; 
 Console.WriteLine(value);
}
else
{
 Console.WriteLine("nullableInt is null");
}

21. Explain how garbage collection works in C#.

Garbage Collection (GC) in C# is an automatic memory management feature. The .NET CLR (Common Language Runtime) automatically reclaims memory that is no longer in use by the application, preventing memory leaks. The GC operates on a generational system.

When an object is created, it's placed in Generation 0. The GC checks memory periodically. If an object in Generation 0 is still in use, it's promoted to Generation 1. Less frequently, Generation 1 objects are checked; survivors move to Generation 2. Objects in Generation 2 have survived multiple garbage collection cycles. This approach optimizes performance because frequently used objects are checked less often. When memory is low, the GC runs, freeing up memory by collecting objects that are no longer referenced. The Dispose() method (and using statement) allows deterministic cleanup for objects holding unmanaged resources, complementing the GC's automatic management of managed memory.

22. What is asynchronous programming in C# and when should you use it?

Asynchronous programming in C# allows your program to initiate a long-running operation without blocking the main thread. This means the UI remains responsive, and the application doesn't freeze while waiting for tasks like network requests, file I/O, or database queries to complete. C# achieves this primarily using the async and await keywords.

You should use asynchronous programming when dealing with operations that may take a significant amount of time, particularly when these operations could potentially block the UI thread. Common scenarios include:

• UI applications: To keep the UI responsive during long operations.
• Web applications: To handle multiple requests concurrently without blocking threads.
• I/O-bound operations: Tasks that involve reading from or writing to files, network streams, or databases.
• CPU-bound operations: Although less common, can improve responsiveness if handled on a separate thread (consider Task.Run with async and await).

23. Describe the purpose of the 'virtual' and 'override' keywords.

The virtual keyword allows a method in a base class to be overridden in a derived class. It declares that derived classes can provide their own specific implementation of the method. Without virtual, the base class implementation is always used.

The override keyword is used in a derived class to provide a new implementation for a virtual method inherited from a base class. It explicitly states that the derived class method is replacing the base class method's functionality. Using override enforces compile-time checking to ensure that the method signature matches the base class's virtual method. It also signals intent, making the code clearer and easier to maintain.

24. What is the difference between a struct and a class?

The primary difference between a struct and a class lies in their default access modifiers. In C++, structs have members that are public by default, while class members are private by default. This means that if you don't explicitly specify an access modifier (public, private, protected) for a member in a struct, it will be accessible from anywhere. Conversely, in a class, members without an explicit access modifier are only accessible from within the class itself or its friends.

In practice, structs are often used to represent simple data structures where the members are intended to be directly accessed, while classes are used for more complex objects with encapsulated data and methods. However, functionally, both structs and classes can have methods, constructors, destructors, and inheritance, making them very similar in capability. The choice between using a struct and a class often depends on the intended use and the desired level of encapsulation. For example:

struct Point {
 int x; // public by default
 int y;
};

class Rectangle {
 int width; // private by default
 int height;
public:
 void setWidth(int w) { width = w; }
 void setHeight(int h) { height = h; }
};

25. Explain the concept of lambda expressions in C#.

Lambda expressions in C# are anonymous functions that can be treated as values. They provide a concise way to create function delegates or expression tree types.

They're typically used for short, inline function definitions, especially in LINQ queries and event handlers. The basic syntax is (input-parameters) => expression. For example, x => x * x is a lambda expression that takes an input x and returns its square. Multiple parameters are comma separated, (x, y) => x + y. Lambda expressions can also have statement bodies enclosed in curly braces, (x) => { return x * 2; }.

26. What are collection initializers in C#?

Collection initializers provide a simplified syntax to initialize collections (like lists, dictionaries, etc.) when they are created. Instead of adding elements one by one using the Add() method, you can specify the initial elements directly within curly braces {}.

For example, consider a list of integers: List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };. Or a dictionary: Dictionary<string, int> ages = new Dictionary<string, int> { { "Alice", 30 }, { "Bob", 25 } };. This makes the code more concise and readable.

Intermediate C# interview questions

1. Explain the difference between `Func<T, TResult>` and `Action<T>` in C#.

Func<T, TResult> and Action<T> are both delegates in C#, but they serve different purposes. Func<T, TResult> represents a method that takes one or more input parameters of type T and returns a value of type TResult. The last generic type parameter always denotes the return type. For example, Func<int, string> represents a function that takes an integer and returns a string.

Action<T> on the other hand, represents a method that takes one or more input parameters of type T but does not return a value (i.e., its return type is void). For example, Action<string> represents a method that takes a string as input and performs some action, but doesn't return anything. In essence, Func is for functions that compute and return a result, while Action is for procedures that perform an operation without returning a value.

2. What are extension methods, and how can they be used? Provide an example.

Extension methods allow you to add new methods to existing types without modifying the original type itself or creating a new derived type. They are special static methods defined in a static class, where the first parameter specifies the type that the method extends, preceded by the this keyword. This enables you to call the extension method as if it were a member of the extended type.

For example, to add a method that counts words in a string:

public static class StringExtensions
{
 public static int WordCount(this string str)
 {
 return str.Split(new char[] { ' ', '.', '?' }, StringSplitOptions.RemoveEmptyEntries).Length;
 }
}

//Usage
string text = "Hello world!";
int count = text.WordCount(); // count will be 2

3. How does the `yield` keyword work in C#, and what problem does it solve?

The yield keyword in C# allows you to create an iterator method that returns a sequence of values one at a time, without having to create a temporary collection to hold the entire result. When the yield return statement is executed, the current value is returned to the caller, and the state of the method is saved. Execution resumes from that point the next time the iterator is called. yield break can be used to terminate the iteration.

The problem yield solves is primarily related to memory efficiency and deferred execution. Instead of generating a complete list in memory before returning it, yield allows you to produce values only when they are requested. This is particularly useful when dealing with large datasets or infinite sequences as you only process elements as needed, thus improving performance and reducing memory footprint.

4. What is the purpose of the `async` and `await` keywords in C#, and how do they work together?

The async and await keywords in C# are used to write asynchronous code more cleanly and readably. async marks a method as asynchronous, allowing the use of await within it. The await keyword pauses the execution of the async method until the awaited task completes, without blocking the calling thread. Execution resumes at the point after the await once the task finishes.

They work together to make asynchronous operations appear more like synchronous code. The compiler transforms the async method into a state machine, handling the callbacks and continuations behind the scenes. This avoids the complex and error-prone callback-based approach to asynchronous programming. The async method must return Task, Task<T>, or void. Using async and await significantly improves the responsiveness of applications, especially those involving I/O-bound operations.

5. Describe the difference between `Task.Run()` and `Task.Factory.StartNew()`.

Task.Run() is a simplified wrapper around Task.Factory.StartNew(). Task.Run() is designed for the most common scenario: queuing work to the thread pool. It automatically infers the appropriate task creation options. Underneath, Task.Run(action) is equivalent to Task.Factory.StartNew(action, CancellationToken.None, TaskCreationOptions.DenyChildAttach, TaskScheduler.Default).

Task.Factory.StartNew() offers more control and flexibility. It allows you to specify parameters such as CancellationToken, TaskCreationOptions, and a custom TaskScheduler. This is useful for advanced scenarios, such as specifying the task's behavior (e.g., LongRunning, AttachedToParent) or running the task on a specific scheduler other than the default thread pool scheduler. Task.Run() is generally preferred for simple fire-and-forget tasks due to its simplicity, while Task.Factory.StartNew() is used when more fine-grained control is required.

6. What is LINQ, and how does it improve code readability and maintainability? Give a basic example.

LINQ (Language Integrated Query) is a set of features in .NET that provides a unified way to query data from various data sources. These sources can include collections (like lists and arrays), databases, XML documents, and more. LINQ allows you to use a consistent syntax to filter, sort, group, and project data, regardless of the underlying data source. This leads to more readable and maintainable code because the same query patterns can be applied across different types of data. It simplifies data access by embedding queries directly into C# or VB.NET code, eliminating the need for separate query languages or APIs.

For example, consider a list of integers. Without LINQ, filtering for even numbers might involve a loop and conditional statements. With LINQ, it's much cleaner:

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6 };
//Using LINQ
IEnumerable<int> evenNumbers = numbers.Where(n => n % 2 == 0);
//Without LINQ
List<int> evenNumbersList = new List<int>();
foreach (int number in numbers)
{
 if (number % 2 == 0)
 {
 evenNumbersList.Add(number);
 }
}

LINQ improves readability because the intent of the query (filtering for even numbers) is immediately clear. It improves maintainability by reducing the amount of boilerplate code required for data manipulation and using common operations, making code easier to understand and modify.

7. Explain the concept of deferred execution in LINQ.

Deferred execution in LINQ means that a query is not executed at the point where it is defined. Instead, the execution is postponed until the query's results are actually needed. This typically happens when you iterate over the results using a foreach loop, convert the results to a list (e.g., using .ToList()), or call a method that requires immediate evaluation (e.g., .Count(), .First(), .Single()).

The main benefit of deferred execution is performance optimization. LINQ providers can optimize the query execution based on how the results are ultimately used. If you only need the first few elements of a large dataset, the entire dataset doesn't need to be processed immediately. Also, it allows you to modify the underlying data source after defining the query but before it is executed, and the query will reflect those changes.

8. Describe the differences between `IEnumerable` and `IQueryable`.

IEnumerable and IQueryable are both interfaces in .NET used for querying data, but they differ significantly in how the data is retrieved and processed. IEnumerable represents a sequence of objects that can be iterated over. When using IEnumerable, the entire data source is loaded into memory, and filtering, sorting, and other operations are performed on the client-side. This means the operations happen after the data is retrieved.

IQueryable, on the other hand, represents a query that can be executed against a data source. It allows you to build up a query expression that is then translated and executed on the data source (e.g., a database). This means filtering, sorting, and other operations are performed on the server-side before the data is retrieved, leading to potentially significant performance gains, especially when dealing with large datasets. The key is that IQueryable uses expression trees to represent the query, which can be analyzed and optimized by the data source.

Key Differences:

• Execution: IEnumerable executes queries in memory (client-side), while IQueryable executes queries on the data source (server-side).
• Data Loading: IEnumerable loads the entire dataset, while IQueryable only retrieves the data that matches the query.
• Performance: IQueryable is generally more efficient for large datasets due to server-side processing.
• Namespace: IEnumerable resides in System.Collections, IQueryable resides in System.Linq.
• IQueryable inherits from IEnumerable.

Example:

// IEnumerable (client-side filtering)
IEnumerable<Product> productsIEnumerable = db.Products.ToList().Where(p => p.Price > 10);

// IQueryable (server-side filtering)
IQueryable<Product> productsIQueryable = db.Products.Where(p => p.Price > 10);

9. What are the benefits of using interfaces in C#? Explain with an example.

Interfaces in C# offer several benefits, primarily around abstraction and loose coupling. They define a contract that classes can implement, ensuring they provide specific functionality. This allows for polymorphism, where different classes can be treated uniformly through the interface.

Consider this example:

interface ISpeak
{
    void Speak();
}

class Dog : ISpeak
{
    public void Speak() { Console.WriteLine("Woof!"); }
}

class Cat : ISpeak
{
    public void Speak() { Console.WriteLine("Meow!"); }
}

//Use
ISpeak animal1 = new Dog();
ISpeak animal2 = new Cat();

animal1.Speak(); // Output: Woof!
animal2.Speak(); // Output: Meow!

Benefits:

• Abstraction: Hide complex implementation details.
• Loose Coupling: Classes implementing the interface are independent.
• Polymorphism: Treat different classes uniformly.
• Multiple Inheritance: A class can implement multiple interfaces, overcoming C#'s lack of multiple class inheritance.

10. Explain the difference between `struct` and `class` in C#.

In C#, the primary difference between struct and class lies in their nature: struct is a value type, while class is a reference type. This means that when you assign a struct to a new variable or pass it as an argument, a copy of the struct is created. With class, only a reference to the object is copied. Another significant difference is that structs are implicitly sealed, meaning they cannot be inherited from, whereas classes can be inherited.

Furthermore, structs have an implicit parameterless constructor that initializes fields to their default values, and you cannot define your own parameterless constructor. Classes, on the other hand, don't have this limitation. Also, structs are typically used for small data structures, while classes are used for more complex objects with behavior. Struct variables are allocated on the stack, while class objects are allocated on the heap.

11. When would you choose to use a `struct` over a `class` in C#?

In C#, use a struct over a class when you need a lightweight data structure that represents a single value, and value-type semantics are desired. struct are value types, so they're copied on assignment, which is beneficial when you want independent copies of your data and avoid unintended side effects. Choose struct for small, immutable types like Point, Rectangle, or simple data containers.

Conversely, prefer class when dealing with more complex objects that have significant behavior, require inheritance, or benefit from reference-type semantics (sharing the same instance in memory). Classes are reference types and allocated on the heap, thus suited for larger objects or situations where object identity matters.

12. What is boxing and unboxing in C#, and what are the performance implications?

Boxing is the process of converting a value type (like int, bool, struct) to a reference type (object). Unboxing is the reverse process, converting a reference type (that was previously boxed) back to its original value type.

The primary performance implication is due to the memory allocation and type checking involved. Boxing requires allocating memory on the heap to store the value type, and unboxing requires type checking to ensure the object being unboxed is compatible with the target value type. These operations can be relatively slow compared to direct operations on value types, especially if they occur frequently in performance-critical code. This usually leads to increased garbage collection overhead.

13. How does the garbage collector work in C#? What is the difference between generations?

The C# garbage collector (GC) automatically manages memory by reclaiming objects that are no longer in use. It works by periodically inspecting the heap, identifying objects that are unreachable from application roots (static variables, local variables on the stack, CPU registers), and freeing the memory they occupy. The GC uses a mark-and-sweep algorithm, enhanced with generations to optimize performance. The GC makes assumptions that recently created objects are likely to be short-lived and older objects are less likely to be garbage.

Generations are a key part of the GC's optimization strategy. Objects are grouped into generations based on their age: Generation 0 (youngest), Generation 1, and Generation 2 (oldest). When the GC runs, it first tries to collect Generation 0. If that doesn't free enough memory, it collects Generation 1 and then Generation 2 if necessary. This approach avoids collecting the entire heap every time, significantly improving performance because collecting younger generations is faster, the assumption being the younger generation has the most dead objects compared to the number of live objects.

14. Explain the purpose of the `using` statement in C# and how it relates to the `IDisposable` interface.

The using statement in C# provides a convenient way to ensure that an object implementing the IDisposable interface is properly disposed of, even if exceptions occur. It essentially wraps the object's usage in a try...finally block, where the Dispose() method is called in the finally block.

When an object implements IDisposable, it typically holds unmanaged resources like file handles, network connections, or database connections. The Dispose() method releases these resources. The using statement guarantees that Dispose() is called when the block is exited, preventing resource leaks. For example:

using (FileStream fs = new FileStream("file.txt", FileMode.Open))
{
  // Use the file stream
}
// fs.Dispose() is automatically called here

15. What are delegates in C#, and how are they used to implement event handling?

Delegates in C# are type-safe function pointers. They allow you to treat methods as objects, which can be passed as arguments to other methods, stored in data structures, and invoked dynamically. They essentially define a type that represents a reference to a method with a specific signature (return type and parameter types).

Delegates are crucial for implementing event handling in C#. Events are a mechanism for a class to notify other classes (or objects) when something of interest happens. This is achieved using delegates. An event is essentially a wrapper around a delegate. When an event occurs, the delegate's invocation list (a list of methods that should be called when the event is raised) is executed. Example:

public delegate void MyEventHandler(object sender, EventArgs e);
public event MyEventHandler MyEvent;

16. Explain the difference between delegates and events in C#.

Delegates are type-safe function pointers, allowing you to treat methods as objects. They define a signature and can hold references to methods that match that signature. You can directly invoke a delegate.

Events, on the other hand, are a mechanism built on top of delegates that provide encapsulation. They prevent direct invocation from outside the class or struct that defines them. Events use delegates internally, but they add restrictions that ensure only the class that declares the event can raise (invoke) it. Subscribers can only add or remove event handlers. Events enforce a publish-subscribe pattern.

17. Describe how you would implement a custom exception in C#.

To implement a custom exception in C#, you create a new class that inherits from the System.Exception class or one of its derived classes (like System.ApplicationException or System.SystemException). It's best practice to provide constructors similar to those found in System.Exception: a default constructor, a constructor that accepts a message string, and a constructor that accepts a message string and an inner exception.

Here's a basic example:

public class MyCustomException : Exception
{
    public MyCustomException() { }
    public MyCustomException(string message) : base(message) { }
    public MyCustomException(string message, Exception inner) : base(message, inner) { }
}

You can then throw this exception like any other exception:

throw new MyCustomException("Something went wrong!");

Consider adding custom properties or methods to hold or handle exception specific data, like an error code, if required.

18. What are attributes in C#, and how can you create and use them?

Attributes in C# are metadata that provide information about types (classes, structs, enums, interfaces, delegates), methods, fields, properties, events, parameters, and other code elements. They are used to add declarative information to your code which can then be read and acted upon at runtime or compile time using reflection. They provide a powerful way to add behavior or modify the compilation process without altering the core logic of your code.

To create a custom attribute, you define a class that inherits from System.Attribute. You then apply the attribute to code elements using square brackets [] followed by the attribute's name and any constructor parameters. Here is an example:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class MyCustomAttribute : Attribute
{
    public string Description { get; set; }

    public MyCustomAttribute(string description)
    {
        Description = description;
    }
}

[MyCustomAttribute("This is a class description")]
public class MyClass
{
    [MyCustomAttribute("This is a method description")]
    public void MyMethod() { }
}

To access attributes at runtime use reflection:

Type type = typeof(MyClass);
MyCustomAttribute attribute = (MyCustomAttribute)Attribute.GetCustomAttribute(type, typeof(MyCustomAttribute));
if (attribute != null)
{
    Console.WriteLine(attribute.Description);
}

19. Explain what reflection is in C#, and provide a scenario where it might be useful.

Reflection in C# allows you to inspect and manipulate types (classes, interfaces, structs, enums, delegates, and events) at runtime. Instead of knowing the type at compile time, you can discover its properties, methods, fields, events, and other members during program execution. Essentially, it's a way for code to examine and modify its own structure and behavior.

A useful scenario is creating a generic object serializer/deserializer. You could use reflection to iterate through the properties of an arbitrary object, extract their values, and then use that information to serialize the object into a format like JSON or XML. Conversely, during deserialization, you could use reflection to create an instance of the object and populate its properties with data extracted from the serialized format. This avoids writing specific serialization/deserialization code for each class.

20. What are generics in C#, and how do they improve type safety and performance?

Generics in C# allow you to define type-safe data structures and algorithms without committing to specific data types. You can define classes, interfaces, methods, delegates, and events that are parameterized by type. The type parameter is specified when the generic type is instantiated or the generic method is called.

Generics improve type safety by ensuring that the compiler enforces type constraints at compile time. This avoids the need for runtime type checking and casting, which can lead to InvalidCastException errors. Generics also improve performance by avoiding boxing and unboxing operations when working with value types. This is because the compiler can generate specialized code for each type used with the generic type or method, resulting in more efficient execution. The below code snippet is an example of a generic list

List<int> numbers = new List<int>();
numbers.Add(1);
numbers.Add(2);

List<string> names = new List<string>();
names.Add("Alice");
names.Add("Bob");

Advanced C# interview questions

1. Explain the differences between `async` and `await` and how they contribute to building responsive applications.

async and await are syntactic sugar built on top of Promises in JavaScript, designed to make asynchronous code easier to read and write. async declares a function as asynchronous, enabling the use of await within it. await pauses the execution of the async function until the Promise it 'awaits' resolves.

By pausing execution instead of blocking the main thread, async/await allows the event loop to continue processing other tasks, like handling user interactions or rendering updates. This non-blocking behavior is crucial for building responsive applications, preventing the UI from freezing during long-running operations like network requests or complex calculations. Instead of .then() and .catch() for handling promises, we can write code in a more synchronous style. For example:

async function fetchData() {
  try {
    const response = await fetch('https://example.com/data');
    const data = await response.json();
    return data;
  } catch (error) {
    console.error('Error fetching data:', error);
  }
}

2. How does the .NET garbage collector work, and what are the different generations of garbage collection?

The .NET garbage collector (GC) automatically manages memory allocation and deallocation for applications. It reclaims memory occupied by objects that are no longer in use. The GC operates based on the principle of generational garbage collection to improve performance. When the GC runs, it determines which objects are no longer being used by the application. It then reclaims the memory occupied by those objects.

.NET GC uses generations to categorize objects based on their age:

• Generation 0: Short-lived objects, such as temporary variables. This generation is collected frequently.
• Generation 1: Objects that have survived a Generation 0 collection. Serves as a buffer between short-lived and long-lived objects.
• Generation 2: Long-lived objects that have survived multiple garbage collection cycles. This generation is collected the least frequently because it contains objects that are likely to remain in memory for the duration of the application's lifetime. A full garbage collection involves all generations (0, 1, and 2).

3. Describe the purpose of the `IDisposable` interface and the `using` statement in C#, and explain how they help manage resources.

The IDisposable interface in C# provides a mechanism to release unmanaged resources (e.g., file handles, network connections, database connections) held by an object. It defines a single method, Dispose(), which should contain the logic to clean up these resources. Classes that implement IDisposable signal that they hold resources that need to be explicitly released.

The using statement simplifies the process of calling Dispose() by ensuring that it is always called, even if exceptions occur within the using block. When an object is created within a using statement, the Dispose() method is automatically called when the block is exited. This promotes deterministic resource management, preventing resource leaks and improving application stability. The syntax of a using statement is using (ResourceType resource = new ResourceType()) { ... }

4. What are delegates and events in C#, and how do they facilitate communication between objects?

Delegates are type-safe function pointers. They hold the reference to a method, allowing you to pass methods as arguments to other methods. Events, on the other hand, are a mechanism for a class (publisher) to notify other classes (subscribers) when something of interest happens. They're based on delegates.

Delegates and events facilitate communication between objects by allowing loose coupling. The publisher doesn't need to know the specific subscribers; it only needs to raise the event. Subscribers register their interest in the event via delegate handlers. When the event is raised, the delegate's invocation list is called, triggering the associated methods in the subscribing objects. This publish-subscribe pattern enables a flexible and extensible way for objects to interact without tight dependencies.

5. Explain the concept of LINQ (Language Integrated Query) and how it simplifies data querying in C#.

LINQ (Language Integrated Query) is a powerful feature in C# that provides a unified way to query data from various sources, such as databases, XML, collections, and more, directly within the C# code. It simplifies data querying by using a consistent syntax, regardless of the data source. Instead of writing different code for each data source, LINQ allows developers to use a standard set of query operators (e.g., Where, Select, OrderBy) and syntax to filter, project, and sort data.

LINQ offers several advantages, including improved code readability and maintainability, reduced code complexity, and compile-time type checking. LINQ queries can be written using either query syntax (similar to SQL) or method syntax (using lambda expressions). For instance:

// Query Syntax
var results = from item in collection where item.Property > 10 select item.Name;

// Method Syntax
var results = collection.Where(item => item.Property > 10).Select(item => item.Name);

6. What are extension methods, and how can they be used to add functionality to existing classes without modifying their source code?

Extension methods allow you to add new methods to existing types (classes, structs, interfaces) without modifying the original type's source code. They are a special kind of static method that can be called as if they were instance methods of the extended type. To define an extension method in C#, you need to:

• Declare a static class.

• Define a static method within that class.

• Use the this keyword as the first parameter of the method, followed by the type you want to extend. For example:

  public static class StringExtensions
  {
      public static bool IsValidEmail(this string str)
      {
          // Email validation logic here
          return str.Contains("@") && str.Contains(".");
      }
  }
  

After defining the extension method, you can call it on any instance of the extended type as if it were a regular instance method: string email = "test@example.com"; bool isValid = email.IsValidEmail();

7. Describe the differences between value types and reference types in C#, and how they affect memory management.

Value types (e.g., int, bool, struct, enum) store their data directly within their memory allocation. When you assign a value type to a new variable, a copy of the data is created. Each variable then has its own independent copy. Reference types (e.g., string, class, array, delegate) store a reference (a memory address) to the actual data stored elsewhere on the heap. When you assign a reference type to a new variable, only the reference is copied, not the underlying data. Both variables then point to the same memory location.

This difference significantly impacts memory management. Value types are typically allocated on the stack, providing fast allocation and deallocation, and are automatically deallocated when they go out of scope. Reference types are allocated on the heap, which requires more overhead for allocation and deallocation. Garbage collection is responsible for reclaiming memory occupied by reference types that are no longer in use, which introduces a performance cost.

8. What is reflection in C#, and how can it be used to inspect and manipulate types at runtime?

Reflection in C# allows you to inspect and manipulate types (classes, interfaces, structures, etc.) at runtime. This means you can discover information about types, create instances of them, invoke their methods, and access their properties, all without knowing their names or structures at compile time. It is primarily achieved through classes found in the System.Reflection namespace.

Reflection can be used for several purposes:

• Inspecting Types: Discovering information like methods, properties, fields, and attributes of a type using methods like GetType(), GetMethods(), GetProperties(), etc.
• Creating Instances: Dynamically creating instances of types using Activator.CreateInstance().
• Invoking Methods: Calling methods of a type dynamically using MethodInfo.Invoke().
• Accessing Properties and Fields: Getting and setting values of properties and fields using PropertyInfo.GetValue(), PropertyInfo.SetValue(), FieldInfo.GetValue(), and FieldInfo.SetValue().

For example, the following code snippet demonstrates the use of reflection:

Type myType = Type.GetType("MyNamespace.MyClass");
object instance = Activator.CreateInstance(myType);
MethodInfo method = myType.GetMethod("MyMethod");
method.Invoke(instance, null);

9. Explain the purpose of attributes in C# and how they can be used to add metadata to code elements.

Attributes in C# are used to add declarative metadata to code elements (assemblies, modules, classes, methods, properties, etc.). They provide a way to associate information with code that can be accessed at runtime using reflection. This metadata can be used for various purposes, such as serialization, validation, code generation, or documentation.

Attributes are defined as classes that inherit from System.Attribute. You apply them to code elements by placing them within square brackets [] before the element. For example, [Obsolete("Use NewMethod instead")] public void OldMethod() { ... } marks the OldMethod as obsolete, with the provided message. Reflection can then be used to read this Obsolete attribute and take appropriate action.

10. What are generics in C#, and how do they enable type-safe programming with reusable code?

Generics in C# allow you to define type-safe data structures and algorithms without committing to a specific data type. They use type parameters (e.g., List<T>) which are placeholders that are replaced with an actual type when the generic type is used. This eliminates the need for casting and reduces the risk of runtime type errors.

Generics enable type-safe programming because the compiler enforces type constraints at compile time. Reusable code is achieved because you can write a single generic class or method that works with multiple data types without code duplication. For instance, a generic list class can store integers, strings, or any other type safely and efficiently, avoiding the type casting associated with object-based collections. For example:

public class GenericList<T>
{
    private T[] items;
    //...
    public T GetItem(int index) { return items[index]; }
}

11. How does the `yield` keyword work in C#, and how can it be used to create iterators?

The yield keyword in C# is used to create iterators in a stateful manner. Instead of returning a collection all at once, a method using yield returns an IEnumerable or IEnumerator that yields elements one at a time as they are requested. When yield return is encountered, the current element is returned, and the method's state is preserved. Execution resumes from that point when the next element is requested.

Using yield, custom iterators can be created easily. For example:

public IEnumerable<int> GetNumbers(int max)
{
    for (int i = 0; i < max; i++)
    {
        yield return i;
    }
}

This avoids needing to create a separate class that implements IEnumerable and IEnumerator manually. yield break can be used to indicate the end of the iteration.

12. Describe the concept of covariance and contravariance in C# generics, and provide examples of their usage.

Covariance and contravariance describe how type parameters in generic interfaces and delegates can be converted implicitly. Covariance allows you to use a more derived type than originally specified (e.g., IEnumerable<string> can be used where IEnumerable<object> is expected if string inherits from object). Contravariance allows you to use a less derived type than originally specified (e.g., an action that accepts object can be used where an action accepting string is expected).

Covariance is supported for generic type parameters declared as out (output), and contravariance is supported for type parameters declared as in (input). For example:

• Covariance: interface ICovariant<out T> {} ICovariant<string> cov = new Covariant<string>(); ICovariant<object> objCov = cov;
• Contravariance: interface IContravariant<in T> {} IContravariant<object> contra = new Contravariant<object>(); IContravariant<string> strContra = contra;

Delegates like Action<T> and Func<T> also support variance based on their parameter types. Action<object> objAction = (o) => {}; Action<string> stringAction = objAction; (contravariance). Func<string> stringFunc = () => "test"; Func<object> objectFunc = stringFunc; (covariance).

13. What are tuples in C#, and how do they provide a way to group multiple values into a single object?

Tuples in C# are a way to group multiple values of potentially different types into a single, lightweight data structure. Before C# 7.0, this was often accomplished with System.Tuple classes, but these were cumbersome due to item naming (Item1, Item2, etc.). C# 7.0 introduced value tuples, which are more efficient and offer named fields.

Value tuples are structures, so they are value types (stored on the stack). They provide a concise syntax for creating and working with composite data. For example, (int age, string name) person = (30, "Alice"); demonstrates a tuple with named fields. You can then access these fields using person.age and person.name. Value tuples enhance code readability and reduce the need to define custom classes or structs for simple data groupings. They are easily created using parenthesis and can be returned as a single return value from a function.

Example:

(int, string) GetPerson() {
 return (30, "Bob");
}

14. Explain the purpose of the `dynamic` keyword in C#, and how it enables late-bound programming.

The dynamic keyword in C# allows you to bypass static type checking at compile time. Variables declared as dynamic have their type determined at runtime. This enables late-bound programming, where method calls and property access are resolved during execution instead of during compilation.

Using dynamic is useful when working with objects whose structure isn't known at compile time, such as when interacting with COM objects, dynamic languages like Python or Ruby, or when using reflection. However, it's crucial to note that errors caused by incorrect usage of dynamic are only detected at runtime, potentially leading to unexpected behavior. Here's an example:

dynamic myVariable = GetUnknownObject();
myVariable.SomeMethod(); // This will compile, but might throw an exception at runtime if SomeMethod doesn't exist

15. What are lambda expressions in C#, and how can they be used to create anonymous functions?

Lambda expressions in C# are a concise way to create anonymous functions. They are essentially unnamed methods that can be treated as data. The syntax is (input parameters) => expression or statement block. For example, x => x * x is a lambda expression that takes an input x and returns its square.

They are used to create delegates or expression tree types. You can assign a lambda expression to a delegate type, such as Func<int, int> or Action<string>. They're very useful in LINQ queries, event handlers, and any scenario where a short, inline function is needed. Example numbers.Where(n => n % 2 == 0) filters even numbers from a list called numbers.

16. Describe the different types of collections available in C#, such as lists, dictionaries, and sets, and explain their use cases.

C# offers various collection types, each suited for different scenarios. Lists (List<T>) are ordered collections that allow duplicate elements and provide access by index. They are ideal when you need to maintain the order of elements and frequently access elements by their position. Dictionaries (Dictionary<TKey, TValue>) store key-value pairs, providing fast lookups based on the key. Use them when you need to quickly retrieve values based on a unique identifier.

Sets (HashSet<T>) are unordered collections that store unique elements. They are useful for checking membership efficiently or removing duplicate entries. Other collections include Queue<T> (FIFO), Stack<T> (LIFO), and specialized collections like ObservableCollection<T> (for data binding) and BlockingCollection<T> (for thread-safe operations). Choosing the right collection depends on the specific requirements of your application, considering factors like ordering, uniqueness, and performance requirements.

17. What is dependency injection (DI) in C#, and how can it be used to improve the testability and maintainability of code?

Dependency Injection (DI) in C# is a design pattern where a class receives its dependencies from external sources rather than creating them itself. This promotes loose coupling between components. Instead of a class creating its dependencies directly (new keyword), these dependencies are "injected" into the class, typically through its constructor, properties, or methods.

DI improves testability because dependencies can be easily replaced with mock objects or stubs during unit testing. This allows you to isolate the code being tested and verify its behavior without relying on external systems. Maintainability is enhanced because changes to one dependency are less likely to affect other parts of the application, due to the loose coupling. DI also promotes code reuse and makes it easier to refactor code.

18. Explain the concept of the `Task` Parallel Library (TPL) in C#, and how it simplifies parallel programming.

The Task Parallel Library (TPL) in C# is a set of classes and APIs in the System.Threading.Tasks namespace that simplifies adding parallelism and concurrency to applications. It abstracts away many of the complexities of working directly with threads, allowing developers to focus on the work that needs to be done rather than the mechanics of thread management. The TPL automatically handles partitioning the work, scheduling threads, managing the thread pool, and handling cancellation, all in an efficient and scalable way. The main component of TPL is the Task object which represents an asynchronous operation.

TPL simplifies parallel programming by providing a higher-level abstraction over threads. Key benefits include:

• Simplified code: Developers can express parallel operations more concisely using Task.Run, Parallel.For, Parallel.ForEach, and async/await keywords.
• Automatic thread management: The TPL handles thread pool management, reducing the overhead of creating and destroying threads manually.
• Exception handling: Provides a centralized way to handle exceptions thrown by tasks.
• Cancellation support: Built-in support for cancelling tasks.
• Data parallelism: Simplifies parallel processing of data collections using Parallel.For and Parallel.ForEach.

19. What are asynchronous streams in C#, and how do they enable the processing of data streams asynchronously?

Asynchronous streams in C# (introduced with IAsyncEnumerable<T> and IAsyncEnumerator<T>) enable the processing of data streams asynchronously. This allows you to iterate through a sequence of data where each element might take some time to produce, without blocking the calling thread. This is particularly useful when dealing with I/O-bound operations like reading from a database, network stream, or file. They help make better use of resources and reduce wait times, particularly for large datasets.

Instead of using a standard foreach loop on an IEnumerable<T>, you use await foreach on an IAsyncEnumerable<T>. Here's a basic example:

async IAsyncEnumerable<int> GenerateNumbers()
{
    for (int i = 0; i < 10; i++)
    {
        await Task.Delay(100); // Simulate an async operation
        yield return i;
    }
}

async Task ProcessNumbers()
{
    await foreach (var number in GenerateNumbers())
    {
        Console.WriteLine(number);
    }
}

Key elements:

• IAsyncEnumerable<T>: Represents an asynchronous sequence of data.
• IAsyncEnumerator<T>: Provides the mechanism to iterate over the asynchronous sequence.
• await foreach: Used to asynchronously iterate over the stream.

20. Describe the purpose of the `HttpClient` class in C#, and how it can be used to make HTTP requests.

The HttpClient class in C# provides a base class for sending HTTP requests and receiving HTTP responses from a resource identified by a URI. It acts as a client to interact with HTTP servers. It simplifies sending various HTTP requests like GET, POST, PUT, DELETE, etc.

To use HttpClient, you create an instance of it, configure it with settings like base address or default headers if needed, and then use its methods (e.g., GetAsync, PostAsync) to send requests. These methods typically return a Task<HttpResponseMessage>, allowing for asynchronous operations. The response message can then be parsed to retrieve the content, headers, and status code. For example:

using System.Net.Http;
using System.Threading.Tasks;

public class Example
{
    public static async Task Main(string[] args)
    {
        using (HttpClient client = new HttpClient())
        {
            client.BaseAddress = new Uri("https://example.com");
            HttpResponseMessage response = await client.GetAsync("/api/data");
            if (response.IsSuccessStatusCode)
            {
                string result = await response.Content.ReadAsStringAsync();
                System.Console.WriteLine(result);
            }
        }
    }
}

21. What are custom attributes, and how would you implement and use them for custom metadata handling in your applications?

Custom attributes, also known as annotations in some languages, are a way to add metadata to code elements like classes, methods, properties, and fields. They allow you to associate information with these elements that can be accessed at runtime using reflection or compile time using source generators.

To implement custom attributes, you first define a class that inherits from the Attribute base class. You can then apply this attribute to code elements using the [AttributeName] syntax. To use the metadata, you would typically use reflection to inspect the code element and retrieve the attribute instance. Here's an example in C#:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class MyCustomAttribute : Attribute
{
    public string Description { get; set; }
    public MyCustomAttribute(string description)
    {
        Description = description;
    }
}

[MyCustomAttribute("This is a sample class")]
public class MyClass {
    [MyCustomAttribute("This is a sample method")]
    public void MyMethod() { }
}

//accessing the Attribute:
Type type = typeof(MyClass);
MyCustomAttribute attribute = (MyCustomAttribute)Attribute.GetCustomAttribute(type, typeof(MyCustomAttribute));
if (attribute != null) {
    Console.WriteLine(attribute.Description);
}

22. Explain different ways to handle errors and exceptions in C#, including try-catch blocks, custom exceptions, and global exception handling.

C# offers several mechanisms for handling errors and exceptions. The primary way is using try-catch blocks. Code that might throw an exception is placed within the try block. If an exception occurs, the control is transferred to the corresponding catch block, where you can handle the exception (e.g., log it, display an error message, or attempt recovery). Multiple catch blocks can handle different types of exceptions. A finally block can be added to ensure that certain code (e.g., releasing resources) always executes, regardless of whether an exception was thrown or caught.

You can also create custom exceptions by inheriting from the Exception class. This allows you to define specific exception types for your application's needs. Finally, global exception handling can be implemented to catch unhandled exceptions at the application level. This usually involves subscribing to events like Application.ThreadException (Windows Forms) or AppDomain.UnhandledException (Console applications) to log or handle exceptions that were not caught by try-catch blocks. This is often used to prevent application crashes and to provide more graceful error reporting. throw; can be used to rethrow the exception so that it can be handled by higher layers.

23. Can you discuss the purpose and benefits of using immutable data structures in C# for concurrent programming?

Immutable data structures in C# offer significant benefits for concurrent programming. Their primary purpose is to prevent race conditions and data corruption by ensuring that once created, the state of the object cannot be changed. This eliminates the need for locks or other synchronization mechanisms when multiple threads access the same data, simplifying concurrent code and improving performance.

The key benefits include:

• Thread Safety: No need for locks, reducing complexity and improving performance.
• Predictability: Easier to reason about the state of the application.
• Reduced Bugs: Eliminates a common source of concurrency errors.
• Simplified Testing: Easier to test concurrent code without race conditions.

For example, using ImmutableList<T> avoids issues compared to List<T> where concurrent modifications without proper locking can lead to unexpected behavior.

24. How does C# support interoperability with unmanaged code, and what are the challenges associated with it?

C# supports interoperability with unmanaged code primarily through Platform Invoke (P/Invoke) and COM Interop. P/Invoke allows C# code to call functions in DLLs written in languages like C or C++. COM Interop enables C# code to interact with COM components. To use P/Invoke, you declare the external function using the DllImport attribute, specifying the DLL name and other details. For COM Interop, you can import COM type libraries as .NET assemblies using tools like tlbimp.exe.

Challenges include:

• Marshalling data: Converting data types between managed (.NET) and unmanaged environments can be complex and error-prone. Incorrect marshalling can lead to crashes or incorrect data.
• Memory management: Unmanaged code often requires manual memory management, which can lead to memory leaks or corruption if not handled carefully.
• Exception handling: Exceptions thrown in unmanaged code are not automatically propagated to the managed environment, requiring explicit handling.
• Security: Interacting with unmanaged code can introduce security vulnerabilities if the unmanaged code is not trusted or contains vulnerabilities.

25. Explain the concepts of multi-threading and parallelism in C#, and how would you implement them to improve performance?

Multi-threading and parallelism are both techniques for achieving concurrency, but they differ in their execution. Multi-threading involves multiple threads running within a single process, sharing the same memory space. This is useful for I/O-bound tasks or tasks that can be broken down into smaller, independent units of work. Parallelism, on the other hand, involves running multiple tasks simultaneously on multiple CPU cores. This is suitable for CPU-bound tasks that can be divided into independent subtasks.

To implement them in C#, you can use the System.Threading namespace for multi-threading and the System.Threading.Tasks namespace, especially the Parallel class, for parallelism. Here's a simple example using Parallel.For for parallelism:

Parallel.For(0, 100, i => {
 // Code to be executed in parallel for each value of i
 Console.WriteLine($"Task {i} running on thread {Thread.CurrentThread.ManagedThreadId}");
});

For multi-threading, you can use the Thread class or the ThreadPool class. For example:

Thread thread = new Thread(() => {
    // Code to be executed in a separate thread
    Console.WriteLine($"Task running on thread {Thread.CurrentThread.ManagedThreadId}");
});
thread.Start();

To improve performance, identify tasks that can run concurrently and choose the appropriate technique (multi-threading or parallelism) based on whether the task is I/O-bound or CPU-bound. Also, consider factors like thread synchronization and data sharing to avoid race conditions and deadlocks.

Expert C# interview questions

1. Explain the nuances of using `async` and `await` in a complex C# application. How do you ensure proper error handling and prevent deadlocks?

Using async and await in complex C# applications involves understanding their impact on thread context and synchronization. async marks a method as asynchronous, allowing the use of await. await suspends execution until an awaited task completes, returning control to the caller without blocking the thread. This is crucial for UI responsiveness and scalability.

Error handling is essential. Use try-catch blocks around await expressions. For example:

async Task MyAsyncMethod()
{
  try
  {
    await SomeTaskAsync();
  }
  catch (Exception ex)
  {
    // Handle the exception
  }
}

To prevent deadlocks, avoid .Result or .Wait() on Task objects in async methods (or methods called from async methods). These block the calling thread, potentially leading to a deadlock if the awaited task tries to access the blocked thread's context. ConfigureAwait(false) can mitigate deadlocks when dealing with UI contexts, allowing the continuation to run on a thread pool thread. Favor async all the way down approach. When an API only provides synchronous methods, consider wrapping them in Task.Run.

2. Describe scenarios where you would use a custom `TaskScheduler` in C#. Explain how it differs from the default scheduler and the benefits it provides.

A custom TaskScheduler in C# is useful when you need fine-grained control over how and where tasks are executed. Scenarios include: limiting concurrency (e.g., ensuring only N tasks run simultaneously), executing tasks on a specific thread (e.g., a UI thread for updating the user interface), or prioritizing tasks based on custom criteria. For instance, a game engine might use a custom scheduler to prioritize rendering tasks over background processing.

The default TaskScheduler (ThreadPoolTaskScheduler) uses the .NET thread pool. A custom scheduler differs by allowing you to dictate the execution environment and order. The benefits are increased control over resource usage, improved responsiveness (by prioritizing critical tasks), and the ability to integrate with specific threading models (like STA for UI applications). Here's an example:

public class LimitedConcurrencyTaskScheduler : TaskScheduler
{
  // Implementation details for managing concurrency
}

3. How does the C# compiler handle closures, and what are the potential pitfalls you should be aware of when using them extensively?

The C# compiler handles closures by generating a class to hold the captured variables. This class, sometimes called a "display class", contains fields for any variables from the outer scope that are used within the lambda expression or anonymous method. When the closure is created, an instance of this display class is created, and the captured variables are stored within it. Subsequent executions of the closure then access these captured variables through the display class instance.

A common pitfall when using closures extensively, especially within loops, is the "captured variable problem". If a loop variable is captured directly, all closures created within the loop will end up referencing the same variable instance. This can lead to unexpected behavior where all closures use the final value of the loop variable, rather than the value at the time each closure was created. To avoid this, create a local variable inside the loop and assign the loop variable's value to it. Capture this local variable instead. for (int i = 0; i < 10; i++) { int temp = i; actions.Add(() => Console.WriteLine(temp)); }

4. Discuss the trade-offs between using structs and classes in C#, focusing on memory allocation, performance, and potential boxing/unboxing issues.

Structs are value types and are allocated on the stack (or inline within containing types), while classes are reference types allocated on the heap. This means structs have faster allocation and deallocation and can lead to better performance for small, short-lived objects. However, copying a struct involves copying all its data, which can be expensive for large structs. Classes only copy the reference. A key performance concern with structs is boxing/unboxing. When a struct needs to be treated as an object (e.g., passed to a method expecting System.Object or an interface implemented by the struct), it's boxed (wrapped) into an object on the heap. Unboxing reverses this, both of which involve performance overhead. Classes don't have this issue as they are already reference types.

In summary, use structs for small, immutable data structures where performance is critical and you want to avoid heap allocation, and boxing is minimized. Use classes for more complex objects with mutable state, inheritance, and when you are passing/returning by reference is preferred. Consider the memory footprint of structs if they are large, or copied frequently, as large struct copies may negate the performance benefits.

5. Explain the internals of the C# garbage collector. How can you profile and optimize your code to reduce garbage collection pressure?

The C# garbage collector (GC) is an automatic memory manager. It reclaims memory occupied by objects that are no longer in use. It's generational, meaning it categorizes objects based on their age. Gen 0 holds short-lived objects, Gen 1 holds objects that survived a Gen 0 collection, and Gen 2 holds long-lived objects. The GC works by identifying the root objects (static variables, objects on the stack) and traversing the object graph to find reachable objects. Unreachable objects are considered garbage and their memory is reclaimed. The GC process involves marking reachable objects, relocating them to compact memory (mostly for gen 0 and 1) and updating references, and sweeping (reclaiming the unreachable objects' memory).

To reduce garbage collection pressure, you can profile your code using tools like the .NET Performance Monitor or Visual Studio's diagnostic tools to identify where allocations occur most frequently. Optimization techniques include: object pooling to reuse objects instead of creating new ones, reducing the lifetime of objects by disposing of them promptly (using using statements or IDisposable), minimizing boxing and unboxing operations (since these create new objects), using structs instead of classes for small, value-type objects, and avoiding excessive string concatenation (use StringBuilder instead). Understanding value types vs reference types is crucial. For instance:

// Bad: Creates many string objects
string result = "";
for (int i = 0; i < 1000; i++)
{
 result += i.ToString();
}

// Good: Uses StringBuilder
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 1000; i++)
{
 sb.Append(i.ToString());
}
string result = sb.ToString();

6. Describe the implementation details of LINQ deferred execution. How does it affect performance and debugging, and how can you optimize LINQ queries?

LINQ's deferred execution means a query isn't executed when you define it. Instead, it's executed when you iterate over the results (e.g., using a foreach loop or calling methods like ToList() or ToArray()). The query expression is translated into an expression tree, which is then executed by the LINQ provider when the results are needed. This allows for optimizations, as the provider can analyze the entire query and choose the most efficient execution strategy. It also enables query composition, where multiple queries can be chained together and executed as a single operation.

Deferred execution can impact performance. If the source data changes between query definition and execution, the results will reflect the changes. Also, repeated iteration over the same query will re-execute it each time, potentially causing performance issues. Debugging can be tricky because the actual query execution happens later, making it difficult to trace the values at the point of definition. To optimize LINQ queries: (1) Use ToList() or ToArray() if you need to cache the results and avoid re-execution. (2) Filter data as early as possible in the query to reduce the amount of data processed. (3) Consider using compiled queries for frequently executed queries, as they can improve performance by pre-compiling the expression tree. (4) Be mindful of the underlying data access technology (e.g., Entity Framework) and optimize database queries accordingly. For example, explicitly specifying which columns to select from the table to avoid fetching unnecessary data.

7. What are the advantages and disadvantages of using immutable data structures in C#? How do you effectively implement them and handle updates?

Immutable data structures in C# offer several advantages: they are inherently thread-safe, simplifying concurrent programming; they improve predictability by preventing unexpected state changes; and they can enhance performance through techniques like structural sharing. However, they also have disadvantages: every modification creates a new object, which can lead to increased memory consumption and garbage collection overhead. Creating and manipulating immutable objects can sometimes be less performant than working with mutable objects directly, especially when dealing with large datasets or frequent updates.

To effectively implement immutable data structures in C#, utilize features like readonly fields and properties with only a getter, preventing external modification. For handling updates, avoid direct modification; instead, create new instances with the desired changes, often using methods that return a modified copy. Libraries like System.Collections.Immutable provide pre-built immutable collections that optimize performance and memory usage through structural sharing. Example: ImmutableList<int> newList = originalList.Add(5);

8. Explain the use cases for different types of collections in C# (e.g., `ConcurrentDictionary`, `ImmutableList`). When should you prefer one over another?

C# offers various collection types optimized for different scenarios. List<T> is a general-purpose, resizable array, suitable when you need ordered storage and frequent access by index, but it's not thread-safe. Dictionary<TKey, TValue> provides fast lookups by key, ideal for key-value pairs, but again, not thread-safe for concurrent access.

ConcurrentDictionary<TKey, TValue> is designed for thread-safe operations in multi-threaded environments, preventing data corruption. ImmutableList<T> creates a new list whenever it is modified, thus preserving previous states. Immutable collections ensure that data isn't changed unintentionally, which can greatly assist in debugging and reasoning about code, particularly in concurrent or multi-threaded scenarios or when you need to ensure the history of your collections remains intact.

9. How does the C# runtime handle dynamic method invocation? What are the performance implications and use cases for dynamic programming in C#?

C# handles dynamic method invocation using the dynamic keyword and the Dynamic Language Runtime (DLR). When a variable is declared as dynamic, the type checking is deferred until runtime. This allows you to call methods and access properties that might not exist at compile time. The DLR uses reflection and other mechanisms to resolve the method call at runtime. If the method is found, it's invoked; otherwise, a RuntimeBinderException is thrown.

Performance implications of dynamic method invocation can be significant. Because type checking and method resolution happen at runtime, it's slower than statically typed code. However, dynamic programming is useful when working with COM objects, scripting languages, or situations where the structure of the object is not known at compile time, such as interacting with external APIs or data sources where the schema might change.

10. Discuss the design patterns applicable for building a scalable and maintainable microservices architecture using C# and .NET.

Several design patterns are beneficial for building a scalable and maintainable microservices architecture with C# and .NET. CQRS (Command Query Responsibility Segregation) can separate read and write operations, allowing each to be scaled and optimized independently. Eventual Consistency complements CQRS, accepting that data across services might not be immediately consistent, improving availability and responsiveness. The Saga pattern manages distributed transactions across multiple services, ensuring data consistency in complex workflows, often implemented using orchestrators or choreographies.

Furthermore, API Gateway provides a single entry point for clients, abstracting the underlying microservices and handling concerns like authentication and rate limiting. Circuit Breaker enhances resilience by preventing cascading failures between services. For discoverability and configuration, patterns like Service Discovery (e.g., using Consul or Eureka, though less common in .NET compared to cloud native solutions like Azure Service Fabric or Kubernetes with their built-in discovery) and Externalized Configuration (e.g., using Azure App Configuration or HashiCorp Vault) are crucial. Consider using MediatR library for implementing in-process messaging between services, which promotes loose coupling. Finally, employing a robust Observability strategy (logging, tracing, metrics) using tools like Application Insights, Prometheus, or Grafana is crucial for monitoring and debugging a distributed system.

11. Explain how you would implement a custom attribute in C# to enforce specific coding standards or perform compile-time validation.

To implement a custom attribute in C# for enforcing coding standards or performing compile-time validation, you first define a class that inherits from System.Attribute. This class will hold the properties related to the validation or standard you want to enforce. For example, if you want to enforce a maximum string length for properties, the attribute could contain a MaxLength property.

Next, apply the custom attribute to the relevant code elements (classes, properties, methods, etc.). During compilation, you can use reflection or an analyzer (Roslyn analyzer) to inspect the code, find the attributes, and perform the desired validation or code generation. For compile-time validation, a Roslyn analyzer is preferable as it provides more robust capabilities to flag code issues directly in the IDE and during the build process. The analyzer will analyze the attributed code elements and report errors or warnings based on the criteria defined in the custom attribute. Example:

[AttributeUsage(AttributeTargets.Property)]
public class StringLengthAttribute : Attribute
{
    public int MaxLength { get; set; }
    public StringLengthAttribute(int maxLength)
    {
        MaxLength = maxLength;
    }
}

public class MyClass
{
    [StringLength(50)]
    public string MyProperty { get; set; }
}

12. Describe the different ways to implement inter-process communication (IPC) in C#. What are the trade-offs of each approach?

C# offers several IPC mechanisms, each with its own advantages and disadvantages. Common methods include:

• Pipes (Named and Anonymous): Pipes enable communication between processes on the same machine. Named pipes allow communication between unrelated processes, while anonymous pipes are typically used for communication between a parent and child process. They are relatively simple to implement but primarily suited for local communication. Performance can be good, but security needs to be considered for named pipes.
• Message Queues: Message queues facilitate asynchronous communication by allowing processes to send and receive messages. They're reliable for scenarios where processes might not be online simultaneously. Implementation involves using libraries or external message queue services (e.g., RabbitMQ). Overhead and complexity are higher than pipes.
• TCP/IP Sockets: Sockets provide a versatile way to communicate over a network (or locally). They offer flexibility for building custom protocols but require more code and consideration of network-related issues like latency and security.
• Memory-Mapped Files: Memory-mapped files allow multiple processes to access the same region of physical memory, offering efficient data sharing. Suitable for large data sets and high-performance scenarios, but require careful synchronization to avoid race conditions. Complexity in setup and memory management can be a drawback.
• gRPC/WebAPI: For distributed applications, gRPC or WebAPI over HTTP(S) are viable options. They provide structured communication using protocols like Protocol Buffers or JSON. While offering interoperability and scalability, these approaches involve more overhead compared to local IPC mechanisms.

13. How would you optimize a C# application for high-throughput and low-latency scenarios? Consider threading, memory management, and network communication.

To optimize a C# application for high-throughput and low-latency, several strategies can be employed. For threading, use the ThreadPool for short-lived tasks or Task based asynchrony (async/await) to avoid blocking the main thread. Minimize lock contention using techniques like lock-free data structures (e.g., ConcurrentDictionary) or fine-grained locking. Consider using Channels for efficient producer-consumer scenarios. For memory management, minimize allocations and deallocations, use object pooling where appropriate, and leverage structs instead of classes for value types where applicable to reduce heap pressure and garbage collection overhead. Profile your code to identify memory leaks and allocation hotspots.

Optimize network communication by using asynchronous sockets for non-blocking I/O. Utilize HttpClient for efficient HTTP requests, setting appropriate connection limits. Serialization can be optimized by using efficient serializers like protobuf-net or System.Text.Json instead of the slower XmlSerializer. Minimize the amount of data transferred over the network by compressing data or only sending necessary information. Consider using techniques like caching to reduce the number of network requests. Profiling using tools like PerfView is key to pinpointing bottlenecks.

14. Explain the advanced features of C# delegates, such as multicast delegates and covariance/contravariance. Provide use case examples.

C# delegates offer advanced features beyond basic function pointers. Multicast delegates allow a delegate instance to hold references to multiple methods. When the delegate is invoked, all the methods in its invocation list are executed sequentially. This is useful for scenarios like event handling, where multiple subscribers need to be notified of an event. For example:

delegate void MyDelegate(string message);

MyDelegate myDel = null;
myDel += Method1;
myDel += Method2;
myDel("Hello"); // Both Method1 and Method2 are called

Covariance and contravariance enable more flexible delegate assignments. Covariance allows a delegate that returns a more derived type to be assigned to a delegate that returns a less derived type. Contravariance allows a delegate that accepts a less derived type as a parameter to be assigned to a delegate that accepts a more derived type as a parameter. These features are supported through the in and out keywords on generic type parameters in delegate declarations. This is particularly useful when working with interfaces and inheritance hierarchies. Example:

delagate TResult MyFunc<out TResult>(); // Covariance example
delagate void MyAction<in T>(T arg); // Contravariance example

15. Describe the implementation and usage of custom iterators in C#. How do they differ from standard `IEnumerable` implementations?

Custom iterators in C# are implemented using the yield return statement within a method, property, or indexer. This allows you to define a state machine that produces a sequence of values on demand, without needing to materialize the entire collection in memory. The method must return IEnumerable<T>, IEnumerator<T>, IEnumerable, or IEnumerator. For example:

public IEnumerable<int> GetNumbers(int limit)
{
    for (int i = 0; i < limit; i++)
    {
        yield return i;
    }
}

Standard IEnumerable implementations typically involve creating a class that implements the interface and stores the entire collection in memory. Custom iterators, on the other hand, defer the generation of elements until they are requested, leading to improved performance and reduced memory consumption, especially when dealing with large or infinite sequences. They differ primarily in their execution strategy, deferred execution versus eager execution of standard collections, and the means by which the collection is stored, either implicitly using yield or explicitly as an object.

16. Explain the concept of Span<T> and Memory<T> in C#. How do they improve performance when working with memory buffers?

Spanand Memoryin C# provide efficient ways to work with contiguous memory regions (buffers) without unnecessary copying, improving performance. Span<T> is a struct representing a contiguous region of memory and offers a safe, zero-allocation abstraction for both managed and unmanaged memory. It's a ref struct, so it lives on the stack and has limitations regarding where it can be stored (e.g., not in class fields). Memory<T> is a struct that wraps a Span<T> or an array and can be stored on the heap.

They improve performance by allowing operations on a portion of a buffer (e.g., a slice of an array or part of a string) without allocating new memory to copy the data. For example, instead of creating a substring by allocating a new string, Span<char> can represent a portion of the original string. This approach reduces memory allocation and garbage collection overhead, particularly beneficial when dealing with large buffers or frequent operations. They enable high-performance scenarios like parsing, data processing, and working with IO streams by providing direct access to memory segments without incurring the cost of copying. They are frequently used with methods like AsSpan() to create a span from existing memory.

ReadOnlySpan<T> and ReadOnlyMemory<T> are the read-only counterparts, ensuring data is not modified.

17. How would you design a C# application to be resilient to transient faults in a distributed environment? Consider using Polly or similar libraries.

To build a resilient C# application in a distributed environment using Polly, I would implement retry policies and circuit breaker patterns to handle transient faults. Retry policies automatically retry failed operations a specified number of times or until a condition is met. The circuit breaker pattern prevents an application from repeatedly trying to execute an operation that is likely to fail, giving the downstream service time to recover. These strategies are implemented using Polly's RetryPolicy and CircuitBreakerPolicy.

For example, consider a scenario where the application communicates with a remote API. I would wrap the API calls with a Polly policy like this:

var retryPolicy = Policy.Handle<HttpRequestException>()
 .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

var circuitBreakerPolicy = Policy.Handle<HttpRequestException>()
 .CircuitBreakerAsync(3, TimeSpan.FromSeconds(30));

var policyWrap = Policy.WrapAsync(retryPolicy, circuitBreakerPolicy);

var result = await policyWrap.ExecuteAsync(() => _httpClient.GetAsync("https://api.example.com/data"));

This ensures that transient network issues or temporary API unavailability doesn't crash the application. Polly handles the retries and circuit breaking based on the defined policies, enhancing the application's resilience. Other policies like Timeout and Fallback could also be considered.

18. Discuss the various ways to serialize and deserialize objects in C#. What are the performance and security considerations for each method?

C# offers several ways to serialize and deserialize objects, each with its own trade-offs. Common methods include: Binary Serialization (using BinaryFormatter), XML Serialization (XmlSerializer), JSON Serialization (using System.Text.Json, Newtonsoft.Json), and Data Contract Serialization (DataContractSerializer). Binary serialization is generally faster but poses significant security risks as it can execute arbitrary code during deserialization; it's also versioning-dependent. XML serialization is more secure and interoperable, but can be less efficient. JSON serialization is widely used due to its human-readable format and broad support, offering good performance, especially with System.Text.Json. Data Contract Serialization provides a balance between performance and flexibility, and it's less sensitive to versioning changes.

Performance depends on the size and complexity of the objects, the chosen serializer, and the underlying implementation. Security-wise, avoid using BinaryFormatter due to its vulnerability to deserialization attacks. Always validate input during deserialization to prevent malicious data from compromising your application. Consider using serialization attributes (e.g., [Serializable], [DataContract]) to control which members are serialized and how. When choosing a method, consider factors such as performance, security, interoperability, and versioning requirements.

19. Explain how you would implement a custom diagnostic source and listener in C# to monitor and diagnose performance issues in a production environment.

To implement a custom diagnostic source and listener in C#, you first define a DiagnosticSource subclass to represent your application's specific events. This involves defining methods that write diagnostic events using DiagnosticSource.Write(), including event names and payloads (anonymous objects or custom classes).

Then, create a DiagnosticListener that subscribes to the DiagnosticSource. The listener's OnNext() method will be invoked whenever the source writes an event. Within OnNext(), you can filter events based on their name and process the payload, for example, by logging the data to a file, sending it to a monitoring system, or triggering alerts. Configure the DiagnosticListener to subscribe to your DiagnosticSource using the DiagnosticListener.AllListeners observable collection or a specific source name. This allows for real-time monitoring and diagnosis of performance issues in production. Example:

//Diagnostic Source
public class MyDiagnosticSource : DiagnosticSource
{
    public override bool IsEnabled(string name) => true; //or custom logic

    public override void Write(string name, object value)
    {
       //write events
    }
}

//Diagnostic Listener
public class MyDiagnosticListener : IObserver<KeyValuePair<string, object>>
{
    public void OnNext(KeyValuePair<string, object> value)
    {
        //Process the event
    }
    //Other IObserver implementation methods
}

20. Describe the role of Roslyn analyzers and code fixes in improving code quality. How can you create custom analyzers for your team's coding standards?

Roslyn analyzers and code fixes play a crucial role in enhancing code quality by enforcing coding standards, identifying potential bugs, and suggesting improvements during development. Analyzers perform static code analysis, examining code for violations of predefined rules, stylistic inconsistencies, or potential runtime issues, offering real-time feedback within the IDE. Code fixes, on the other hand, provide automatic or semi-automatic solutions to address the issues flagged by the analyzers, allowing developers to quickly correct their code and adhere to best practices.

Creating custom Roslyn analyzers involves:

• Defining diagnostic rules that represent the coding standards. These rules specify what to look for in the code and the severity of the issue.
• Implementing an analyzer that detects violations of these rules by traversing the code's syntax tree.
• Writing a code fix provider that suggests and applies code transformations to correct the violations. The analyzer and code fix are typically packaged as a NuGet package for easy distribution and consumption within a team or organization. Example code: DiagnosticDescriptor Rule = new DiagnosticDescriptor(DiagnosticId, Title, MessageFormat, Category, DiagnosticSeverity.Warning, isEnabledByDefault: true, description: Description);

21. How would you implement a generic retry mechanism in C# that handles different types of exceptions and implements exponential backoff?

A generic retry mechanism in C# can be implemented using a combination of delegates, exception filters, and a loop with exponential backoff. The core idea is to define a method that encapsulates the operation to be retried and execute it within a try-catch block.

Here's a simplified example:

public static T Retry<T>(Func<T> operation, int maxRetries = 3, TimeSpan? initialDelay = null, params Type[] exceptionTypes) {
    initialDelay = initialDelay ?? TimeSpan.FromSeconds(1);
    exceptionTypes = exceptionTypes ?? new Type[] { typeof(Exception) };
    int retryCount = 0;
    while (true) {
        try {
            return operation();
        } catch (Exception ex) when (exceptionTypes.Any(type => type.IsInstanceOfType(ex))) {
            if (retryCount >= maxRetries) throw;
            retryCount++;
            TimeSpan delay = initialDelay.Value * Math.Pow(2, retryCount - 1);
            Thread.Sleep(delay);
        }
    }
}

This Retry method takes a function (Func<T> operation) representing the code to be retried, a maximum number of retries, an optional initial delay, and a list of exception types to handle. The while loop executes the operation, catching exceptions that match the provided types. Exponential backoff is achieved by doubling the delay after each retry. If the maximum number of retries is reached, the exception is re-thrown. If the operation is successful, the result is returned.

22. Explain how you can leverage C# features like source generators to automate repetitive tasks and reduce boilerplate code in your projects.

Source generators in C# allow you to inspect user code and generate additional C# source code during compilation. This is powerful for automating repetitive tasks. For example, you could automatically generate implementations for interfaces, create boilerplate code for data transfer objects (DTOs), or generate efficient serialization/deserialization logic. By moving this logic into a generator, you avoid writing and maintaining the same code across multiple projects, reducing boilerplate and the risk of errors.

Specifically, source generators can analyze code marked with custom attributes, interfaces or naming conventions and then generate associated code. They work at compile time, improving performance compared to runtime reflection-based approaches. Some common examples include generating code for INotifyPropertyChanged implementations, dependency injection containers, or automatically creating mapping code between different types. The generated code is added to the compilation, so it's available just like hand-written code.

C# MCQ

public class Animal
{
    public virtual string MakeSound() { return "Generic animal sound"; }
}

public class Dog : Animal
{
    public override string MakeSound() { return "Woof!"; }
}

public class Cat : Animal
{
    public override string MakeSound() { return "Meow!"; }
}

public class Program
{
    public static void Main(string[] args)
    {
        Animal myAnimal = new Cat();
        Console.WriteLine(myAnimal.MakeSound());
    }
}

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.Where(n => n % 2 == 0);
numbers.Add(6);

foreach (var number in evenNumbers)
{
    Console.WriteLine(number);
}

using System;
using System.Threading.Tasks;

public class Example
{
    public static async Task Main(string[] args)
    {
        Console.WriteLine(await GetValueAsync());
    }

    public static async Task<int> GetValueAsync()
    {
        Task<int> task = Task.Run(() => GetValue());
        return task.Result; 
    }

    public static int GetValue()
    {
        return 42;
    }
}

try
{
    // Some code that might throw exceptions
    string str = null;
    int length = str.Length;
}
catch (ArgumentNullException ex)
{
    Console.WriteLine("ArgumentNullException caught");
}
catch (NullReferenceException ex)
{
    Console.WriteLine("NullReferenceException caught");
}
catch (Exception ex)
{
    Console.WriteLine("Exception caught");
}

Dictionary<string, int> myDict = new Dictionary<string, int>();
myDict.Add("apple", 1);

// What will happen if you try to add another key/value pair where the key already exists?
myDict.Add("apple", 2);

Which C# skills should you evaluate during the interview phase?

Assessing a candidate's C# skills in a single interview is challenging, as you can’t possibly evaluate everything. However, focusing on core C# skills ensures you identify candidates with the right expertise. These skills are essential for success in C# development roles.

Object-Oriented Programming (OOP)

Object-Oriented Programming is a important skill. You can use a C# test that includes relevant MCQs to quickly assess a candidate's understanding of OOP concepts, such as inheritance, polymorphism, and encapsulation. This can save time and help filter candidates.

To assess their practical understanding, you can ask a targeted interview question.

Describe the difference between inheritance and composition. When would you use one over the other?

Look for a response that shows they understand the benefits of both. Ideally, they should mention that composition allows for greater flexibility and avoids the 'fragile base class' problem.

.NET Framework and Core

Assessing their .NET knowledge can be done effectively with a .NET assessment. These tests cover the relevant features, libraries, and tools within the .NET framework, helping you filter out candidates with a solid foundation.

A targeted interview question can further reveal their depth of understanding.

Explain the difference between .NET Framework and .NET Core (now .NET). What are the key advantages of .NET Core?

The candidate should mention cross-platform capabilities, modularity, and performance improvements. Their answer should show an awareness of the evolution of the .NET ecosystem.

Asynchronous Programming

You can use a C# assessment to filter candidates on this particular skill. You can evaluate candidates on concepts such as async/await and Task Parallel Library.

To gauge their practical experience, pose a question that requires them to explain the concept.

Describe a scenario where you would use asynchronous programming in a C# application. Explain how async and await keywords work in that context.

Look for an understanding of how asynchronous operations prevent blocking the main thread. They should demonstrate an ability to describe a real-world use case, such as handling network requests or performing background tasks.

LINQ (Language Integrated Query)

Assess their LINQ skills with a targeted LINQ assessment. Such a test can quickly determine if they grasp the syntax and concepts of LINQ, aiding in filtering candidates effectively.

Ask a question that prompts them to explain the purpose of LINQ.

Explain what LINQ is and provide an example of how you've used it in a project.

The candidate should demonstrate an understanding of LINQ's purpose in querying data. Look for a clear example of how they've applied LINQ to simplify data access and manipulation.

Hire Skilled C# Developers with Targeted Assessments and Insightful Interviews

When hiring C# developers, it's important to accurately assess their skills to ensure they meet your project requirements. Verifying their expertise in C# concepts and development practices is the first step.

The most reliable way to evaluate a candidate's C# proficiency is through skills tests. Explore Adaface's range of assessments, including the C# Online Test and the .NET Online Test, to identify top talent.

After testing, you can easily filter and shortlist the most promising candidates. This allows you to focus your interview efforts on those who have demonstrated the strongest C# skills.

Ready to streamline your C# developer hiring process? Sign up for a free trial at Adaface or learn more about our Coding Tests.

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