Top LINQ interview questions and answers - LINQ interview questions and answers for freshers and experienced
Explain what is LINQ? Why is it required?
Language Integrated Query or LINQ is the collection of standard query operators which provides query facilities into.NET framework language like C#, VB.NET.
LINQ is required as it bridges the gap between the world of data and world of objects.

What are the types of LINQ?
  • LINQ to Objects
  • LINQ to XML
  • LINQ to Dataset
  • LINQ to SQL
  • LINQ to Entities
Explain how LINQ is useful than Stored Procedures?
  • Debugging: It is difficult to debug a stored procedure but as LINQ is part of.NET, visual studios 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
List out the three main components of LINQ? Explain what is the extension of the file, when LINQ to SQL is used?
Three main components of LINQ are
Standard Query Operators
Language Extensions
LINQ Providers
The extension of the file used is .dbml
Define what is Where clause and Let clause?
  • Where clause: It allows adding some conditional filters to the query.
  • Let clause: It allows defining a variable and assigning it a value calculated from the data values.
Explain why SELECT clause comes after FROM clause in LINQ?
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.

Explain what is the use of System.XML.Xlinq.dll?
System.Data.Dlinq.dll provides the functionality to work with LINQ to SQL

Explain what is lambda expressions in LINQ?
Lambda expression is referred as a unique function use to form delegates or expression tree types, where right side is the output and left side is the input to the method. For writing LINQ queries particularly, Lambda expression is used.

Explain how LINQ with databases can be used?
LINQ supports XML, SQL, Dataset and Objects. Through LINQ to objects or LINQ to Datasets one can use LINQ with other databases. The objects and datasets take care of database particular operations, and LINQ only needs to deal with those objects and not the database operations directly.

Explain what is the difference between Skip() and SkipWhile() extension method?
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

In LINQ how will you find the index of the element using where () with Lambda Expressions?
In order to find the index of the element using where () with the lambda expression
Where ( ( i, ix ) => i == ix);

Explain how you can assign a lambda expression to a delegate?
To assign a lambda expression to a delegate
Delegate int del (int i);
Del myDelegate=x=>x*x;
Intj = myDelegate (4); //j=16

Explain what is the difference between Statement Lambda and Expression Lambda?
  • Expression Lambdas are extensively used in the construction of Expression Trees
  • To create expression trees statement lambdas cannot be used
Mention what is the role of DataContext classes in LINQ?
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.

Explain what are LINQ query expressions?
Query expression is nothing but an LINQ query. It is a combination of query clauses that identifies the data sources for a query. It contains information for sorting, filtering, grouping or joining to apply to the source data. It determines what information should be retrieved from the data source.CV.

Explain what are compiled queries?
In compiled LINQ queries, the plan is cached in a static class and static class is a global cache. Rather than preparing the query plan from scratch, LINQ prepares plan using stating class object.

Explain how standard query operators useful in LINQ?
Standard Query Operators useful in LINQ are
  • Get a total count of elements in the collection
  • Order the results of a collection
  • Grouping
  • Computing average
  • Joining two collections based on matching keys
  • Filter the results
Explain what is the purpose of LINQ providers in LINQ?
LINQ providers are set of classes that take an LINQ query which generates method that executes an equivalent query against a particular data source.

Explain how you can retrieve a single row with LINQ?
To retrieve a single row with LINQ we need
Public User GetUser (string userName)
DBNameDataContext myDB = new DBNameDataContext ( ) ;
User user = myDB. Users. Single ( u, u.UserName => userName );
Return user;

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

Explain what is “LINQ to Objects”?
When LINQ queries any IEnumerable(Of T) 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.”

Explain how you can differentiate between Conversion Operator “ToDictionary” and “IEnumerable” of LINQ?
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 <T>.

What is PLINQ?
PLINQ stands for Parallel Language Integrated Query. It is the parallel implementation of LINQ, in which a query can be executed by using multiple processors. PLINQ ensures the scalability of software on parallel processors in the execution environment. It is used where data grows rapidly, such as in telecom industry or where data is heterogeneous.
PLINQ also supports all the operators of LINQ. In addition, you can query 'collections by using PLINQ. It can also run several LINQ queries simultaneously and makes use of the processors on the system. Apart from this, PLINQ uses parallel execution, which helps in running the queries quickly. Parallel execution provides a major performance improvement to PLINQ over certain types of legacy code, which takes too much time to execute.

What are the different Visual Basic features that support LINQ?
Visual Basic includes the following features that support LINQ:
  • Anonymous types - Enables you to create a new type based on a query result.
  • Implicitly typed variables - Enables the compiler to infer and assign a type when you declare and initialize a variable.
  • Extension method - Enables you to extend an existing type with your own methods without modifying the type itself.
What is the function of the DISTINCT clause in a LINQ query?
The DISTINCT clause returns the result set without the duplicate values.

What is the DataContext class and how is it related to LINQ?
After you add a LINQ to SQL Classes item to a project and open the O/R Designer, the empty design surface represents an empty DataContext class ready to be configured. The DataContext class is a LINQ to SQL class that acts as a conduit between a SQL Server database and the LINQ to SQL entity classes mapped to that database. This class contains the connection string information and the methods for connecting to a database and manipulating the data in the database. It is configured with connection information provided by the first item that is dragged onto the design surface.

What is the difference between the Take and Skip clauses?
The Take clause returns a specified number of elements. For example, you can use the Take clause to return two values from an array of numbers. The Skip clause skips the specified number of elements in the query and returns the rest. For example, you can use the Skip clause to skip the first four strings in an array of strings and return the remaining array of string.

What is Object Relational Designer (0/R Designer)?
The 0/R Designer provides a visual design surface to create LINQ to SQL entity classes and associations (relationships) that are based on objects in a database.
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