Essential 40+ LINQ Interview Questions and Answers in 2021

The sequence is a collection class on which you want to query. An Element is a single item in the collection class and the class must implement the IEnumerable interface.

Query operators take in the sequence as input, process it and return the new result sequence.

In VB, an LINQ query is executed in the For Each Statement, and in the foreach statement for C#

Extension methods are static functions of a static class. These methods can be invoked similar to the syntax of the instance method. These methods are used when a class needn’t be modified.

Anonymous types are types that are generated by the compiler upon runtime. We don’t need to specify a name when we create an anonymous type. Only properties names are created and values assigned to them at runtime.

In LINQ, Anonymous types are used to also save intermediate results. However, Anonymous types cannot implement interfaces, specify methods, or define static methods.

All defined properties must be initialised and only public fields can be defined.

An Anonymous function is a function without a name. In an Anonymous function, we only define parameters and write the code in curly braces.

Lambda expression is a shortcut to writing delegates. Lambda expression is used to write inline functions that can be passed as arguments to a function or returned as arguments from a function.

The syntax of a Lambda expression is: We specify the input parameters on the left, the lambda operator in the middle and expression or statement block on the right side.

Action refers to the general delegates belonging to the base class library of .NET. We can store only methods with input parameters and void return types in Action. Upto 16 parameters can be specified.

The Predicate delegate is provided by the base class of the .NET library. In Predicate, we are allowed to store only methods with one input parameter and bool return type.

Predicate delegates are useful in scenarios where filtering is needed, say of a list and so on.

• It is very difficult to debug Stored Procedures but relatively easier to debug LINQ’s queries with Visual Studio’s debugger.
• For stored procedures, we need to provide an additional script for deployment but with LINQ, deployment easy because all the code gets compiled into one DLL.
• LINQ is Type Safe. Hence, query errors are checked at compile time.

• We can run strongly typed queries on Datasets by using LINQ.
• If you want to combine the values of 2 datasets, or extract a unique value from a dataset, LINQ will be useful.
• With LINQ, you can query a dataset more effectively with more features when compared to ADO. NET.

A LINQ query expression has a combination of instructions for filtering, sorting, joining or grouping for the source data.

LINQ query expression syntax is very similar to the SQL syntax.

PLINQ is Parallel Language Integrated Query. In PLINQ, a query can be executed using many processors. With PLINQ, Software can be made scalable across environments during execution. Apart from supporting all operators in LINQ, PLINQ also runs many LINQ queries simultaneously as well.

The Take clause is used to return only a specified number of elements. The Skip clause on the other hand, skips a specified number of elements and returns the rest only.

• LINQ to Objects
• LINQ to XML
• LINQ to Dataset
• LINQ to SQL
• LINQ to Entities

Three main components of LINQ are

• Standard Query Operators
• Language Extensions
• LINQ Providers

When LINQ queries any IEnumerable() collection or IEnumerable directly without the use of an intermediate LINQ provider or API such as LINQ to SQL or LINQ to XML is referred as LINQ to Objects.

DataContext class acts as a bridge between SQL Server database and the LINQ to SQL. For accessing the database and also for changing the data in the database, it contains connections string and the functions. Essentially a DataContext class performs the following three tasks:

• Create connection to database.
• It submits and retrieves object to database.
• Converts objects to SQL queries and vice versa.

In order to find the index of the element use the overloaded version of where() with the lambda expression:

where(( i, ix ) => i == ix);

• Debugging: It is difficult to debug a stored procedure but as LINQ is part of.NET, visual studio debugger can be used to debug the queries
• Deployment: For stored procedure, additional script should be provided but with LINQ everything gets compiled into single DLL hence deployment becomes easy
• Type Safety: LINQ is type safe, so queries errors are type checked at compile time

Where clause: It allows adding some conditional filters to the query.

Linq to XML provides the integral document modification capabilities of the DOM and supports LINQ Queries. Using it, we can amend query, find the way, and save the changes of an XML document. It permits us to write queries to reclaim and steer a compilation of elements and attributes.

In compiled queries, the graph is cached in a fixed class, and the fixed level is a worldwide cache. Rather than preparing the question plan from scratch, Linq prepares map using the stating class object.

If you want to use LINQ in an ASP.NET webpage, then LinqDataSource is an important part of the dataset. It is used to set the properties in the markup text, control, retrieve, and modify data. It can also be used to declaratively bind other ASP.NET controls on a page to a data source. In this manner, it is similar to SQL Datasource and ObjectDataSource controls.

With other programming language and C#, LINQ is used, it requires all the variables to be declared first. “FROM” clause of LINQ query defines the range or conditions to select records. So, FROM clause must appear before SELECT in LINQ.

• LINQ to SQL only supports 1 to 1 mapping of database tables, views, sprocs and functions available in Microsoft SQL Server. It's a great API to use for quick data access construction to relatively well designed SQL Server databases. LINQ2SQL was first released with C# 3.0 and .Net Framework 3.5.

• LINQ to Entities (ADO.Net Entity Framework) is an ORM (Object Relational Mapper) API which allows for a broad definition of object domain models and their relationships to many different ADO.Net data providers. As such, you can mix and match a number of different database vendors, application servers or protocols to design an aggregated mash-up of objects which are constructed from a variety of tables, sources, services, etc. ADO.Net Framework was released with the .Net Framework 3.5 SP1.

The difference between First() and Take() is that First() returns the element itself, while Take() returns a sequence of elements that contains exactly one element. (If you pass 1 as the parameter).

• Use First() when you know or expect the sequence to have at least one element. In other words, when it is an exceptional occurrence that the sequence is empty.
• Use FirstOrDefault() when you know that you will need to check whether there was an element or not. In other words, when it is legal for the sequence to be empty. You should not rely on exception handling for the check. (It is bad practice and might hurt performance).
• First() will throw an exception if there's no row to be returned, while FirstOrDefault() will return the default value (NULL for all reference types) instead.

A set of extension methods forming a query pattern is known as LINQ Standard Query Operators. As building blocks of LINQ query expressions, these operators offer a range of query capabilities like filtering, sorting, projection, aggregation, etc.

An Expression Tree is a data structure that contains Expressions, which is basically code. So it is a tree structure that represents a calculation you may make in code. These pieces of code can then be executed by "running" the expression tree over a set of data.

In LINQ, expression trees are used to represent structured queries that target sources of data that implement IQueryable. For example, the LINQ provider implements the IQueryable interface for querying relational data stores. The C# compiler compiles queries that target such data sources into code that builds an expression tree at runtime. The query provider can then traverse the expression tree data structure and translate it into a query language appropriate for the data source.

There may be scenario where we need to execute a particular query many times and repeatedly. LINQ allows us to make this task very easy by enabling us to create a query and make it compiled always. Benefits of Compiled Queries:

• Query does need to compiled each time so execution of the query is fast.
• Query is compiled once and can be used any number of times.
• Query does need to be recompiled even if the parameter of the query is being changed.

In a query expression, it is sometimes useful to store the result of a sub-expression in order to use it in subsequent clauses. You can do this with the let keyword, which creates a new range variable and initializes it with the result of the expression you supply.

• Skip() : It will take an integer argument and from the given IEnumerable it skips the top n numbers
• SkipWhile (): It will continue to skip the elements as far as the input condition is true. It will return all remaining elements if the condition is false.

To solve the conversion type problems “IEnumerable” and “ToDictionary” conversion operator are used.

• “ToDictionary” conversion operator is the instance of Dictionary (k, T). The “keySelector” predicate recognizes the key of each item, while “elementSelector”, is used to extract each single item, if it is given.
• Extension method on “IEnumerable” is.AsEnumerable. AsEnumerable simply returns the source sequence as an object of type IEnumerable .

• Expression Lambdas are extensively used in the construction of Expression Trees.
• To create expression trees statement lambdas cannot be used.

System.Data.Dlinq.dll provides the functionality to work with LINQ to SQL

Linq queries can be scripted for the classes that execute the IEnumerable interface. The system. Linq namespace provides many classes for interfaces need for Linq queries.

The object-relational designer provides a visual design surface to make Linq to SQL entity classes and relations that are based on objects in a database.

The Groupby clause groups the fundamentals that share an ordinary attribute.

The AsParallel extension process is essential to run a parallel query in PLINQ.

Deferred execution is defined as the assessment of expression is postponed until its realized value is required. Deferred execution can significantly improve presentation when you have to influence large data collections, especially in programs that enclose a series of chained queries.

The SqlMetal.exe command-line tool makes code and maps the Linq to SQL constituent.

In LINQ, Select() and SelectMany() are projection operators. The use of a Select() operator is to select a value from a collection whereas the use of SelectMany() operator is to select values from a group of collection, i.e. a nested collection.

Entity classes are the fundamental building blocks of systems developed on LINQ. In this, the entity classes, which are basically an object wrapper for any database table, have a stereotype of the entity.

Quantifier operations in LINQ are those which return a Boolean value, i.e. True or False on execution.

Here are the type of Quantifier Operations in LINQ: - All() - Any() - Contains()