35 LINQ Interview Questions and Answers

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 the world of objects.

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

Anonymous types are types that are generated by compiler at run time. When we create a anonymous type we do not specify a name. We just write properties names and their values. Compiler at runtime create these properties and assign values to them.

var k = new { FirstProperty = "value1", SecondProperty = "value2" };
Console.WriteLine(k.FirstProperty);

Anonymous class is useful in LINQ queries to save our intermediate results.

There are some restrictions on Anonymous types as well:

• Anonymous types can not implement interfaces.
• Anonymous types can not specify any methods.
• We can not define static members.
• All defined properties must be initialized.
• We can only define public fields.

• 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

When LINQ queries any IEnumerable(<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.

LINQ providers are set of classes that take an LINQ query which generates method that executes an equivalent query against a particular data source.

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.

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

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

Three main components of LINQ are

• Standard Query Operators
• Language Extensions
• LINQ Providers

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.

Extension methods are static functions of a static class. These methods can be invoked just like instance method syntax. These methods are useful when we can not want to modify the class. Consider:

public static class StringMethods
{
    public static bool IsStartWithLetterM(this string s)
    {
        return s.StartsWith("m");
    }
}
class Program
{
    static void Main(string[] args)
    {
        string value = "malslfds";
        Console.WriteLine(value.IsStartWithLetterM()); //print true;
 
        Console.ReadLine();
    }
}

An Anonymous function is a special function which does not have any name. We just define their parameters and define the code into the curly braces.

Consider:

delegate int func(int a, int b);
static void Main(string[] args)
{
    func f1 = delegate(int a, int b)
    {
        return a + b;
    };
 
    Console.WriteLine(f1(1, 2));
}

Stored procedures:

(++)

• Great flexibility
• Full control over SQL
• The highest performance available

(--)

• Requires knowledge of SQL
• Stored procedures are out of source control (harder to test)
• Substantial amount of "repeating yourself" while specifying the same table and field names. The high chance of breaking the application after renaming a DB entity and missing some references to it somewhere.
• Slow development

ORM (EF, L2SQL, ADO.NET):

(++)

• Rapid development
• Data access code now under source control
• You're isolated from changes in DB. If that happens you only need to update your model/mappings in one place.

(--)

• Performance may be worse
• No or little control over SQL the ORM produces (could be inefficient or worse buggy). Might need to intervene and replace it with custom stored procedures. That will render your code messy (some LINQ in code, some SQL in code and/or in the DB out of source control).
• As any abstraction can produce "high-level" developers having no idea how it works under the hood

In practice, they mean essentially the same thing. However, it's preferable to use the term deferred.

• Lazy means "don't do the work until you absolutely have to."
• Deferred means "don't compute the result until the caller actually uses it."

When the caller decides to use the result of an evaluation (i.e. start iterating through an IEnumerable<T>), that is precisely the point at which the "work" needs to be done (such as issuing a query to the database).

The term deferred is more specific/descriptive as to what's actually going on. When I say that I am lazy, it means that I avoid doing unnecessary work; it's ambiguous as to what that really implies. However, when I say that execution/evaluation is deferred, it essentially means that I am not giving you the real result at all, but rather a ticket you can use to claim the result. I defer actually going out and getting that result until you claim it.

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.

Consider:

var names = new string[] { "Dog", "Cat", "Giraffe", "Monkey", "Tortoise" };
var result =
    from animalName in names
    let nameLength = animalName.Length
    where nameLength > 3
    orderby nameLength
    select animalName; 

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.

LINQ standard query operators can be categorized into the following ones on the basis of their functionality.

• Filtering Operators (Where, OfType)
• Join Operators (Join, GroupJoin)
• Projection Operations (Select, SelectMany)
• Sorting Operators (OrderBy, ThenBy, Reverse, ...)
• Grouping Operators (GroupBy, ToLookup)
• Conversions (Cast, ToArray, ToList, ...)
• Concatenation (Concat)
• Aggregation (Aggregate, Average, Count, Max, ...)
• Quantifier Operations (All, Any, Contains)
• Partition Operations (Skip, SkipWhile, Take, ...)
• Generation Operations (DefaultIfEmpty, Empty, Range, Repeat)
• Set Operations (Distinct, Except, ...)
• Equality (SequenceEqual)
• Element Operators (ElementAt, First, Last, ...)

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.

Consider:

static class MyCompliedQueries {
	public static Func <DataClasses1DataContext, IQueryable <Person>> CompliedQueryForPerson = 
        CompiledQuery.Compile((DataClasses1DataContext context) = >from c in context.Persons select c);
}

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.

• 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.

Assuming you have some easily-identifiable condition, you can do something like this, which will work for any IEnumerable<T>:

var query = source.Select((value, index) => new { value, index })
                  .Where(x => x.value => Condition(value))
                  .Select(x => x.index)
                  .Take(n);

The important bits are that you use the overload of Select to get index/value pairs before the Where, and then another Select to get just the indexes after the Where... and use Take to only get the first n results.

Consider:

authorsList.RemoveAll(x => x.FirstName == "Bob");

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<T>. For example, the LINQ provider implements the IQueryable<T> 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.

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.

• 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.