Queryable.Select Method

Definition

Namespace:

System.Linq

Assemblies:

netstandard.dll, System.Linq.Queryable.dll

Projects each element of a sequence into a new form.

Overloads

Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>)	Projects each element of a sequence into a new form by incorporating the element's index.
Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>)	Projects each element of a sequence into a new form.

Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>)

Source:

Queryable.cs

Projects each element of a sequence into a new form by incorporating the element's index.

C#

public static System.Linq.IQueryable<TResult> Select<TSource,TResult>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,int,TResult>> selector);

Type Parameters

TSource

The type of the elements of source.

TResult

The type of the value returned by the function represented by selector.

Parameters

source

IQueryable<TSource>

A sequence of values to project.

selector

Expression<Func<TSource,Int32,TResult>>

A projection function to apply to each element.

Returns

IQueryable<TResult>

An IQueryable<T> whose elements are the result of invoking a projection function on each element of source.

Exceptions

ArgumentNullException

source or selector is null.

Examples

The following code example demonstrates how to use Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) to project over a sequence of values and use the index of each element in the projected form.

C#

string[] fruits = { "apple", "banana", "mango", "orange",
                      "passionfruit", "grape" };

// Project an anonymous type that contains the
// index of the string in the source array, and
// a string that contains the same number of characters
// as the string's index in the source array.
var query =
    fruits.AsQueryable()
    .Select((fruit, index) =>
                new { index, str = fruit.Substring(0, index) });

foreach (var obj in query)
    Console.WriteLine("{0}", obj);

/*
    This code produces the following output:

    { index = 0, str =  }
    { index = 1, str = b }
    { index = 2, str = ma }
    { index = 3, str = ora }
    { index = 4, str = pass }
    { index = 5, str = grape }
*/

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) method generates a MethodCallExpression that represents calling Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) depend on the implementation of the type of the source parameter. The expected behavior is that it invokes selector on each element of source to project it into a different form.

Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>)

Source:

Queryable.cs

Projects each element of a sequence into a new form.

C#

public static System.Linq.IQueryable<TResult> Select<TSource,TResult>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,TResult>> selector);

Type Parameters

TSource

The type of the elements of source.

TResult

The type of the value returned by the function represented by selector.

Parameters

source

IQueryable<TSource>

A sequence of values to project.

selector

Expression<Func<TSource,TResult>>

A projection function to apply to each element.

Returns

IQueryable<TResult>

An IQueryable<T> whose elements are the result of invoking a projection function on each element of source.

Exceptions

ArgumentNullException

source or selector is null.

Examples

The following code example demonstrates how to use Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) to project over a sequence of values.

C#

List<int> range =
    new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// Project the square of each int value.
IEnumerable<int> squares =
    range.AsQueryable().Select(x => x * x);

foreach (int num in squares)
    Console.WriteLine(num);

/*
    This code produces the following output:

    1
    4
    9
    16
    25
    36
    49
    64
    81
    100
*/

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) method generates a MethodCallExpression that represents calling Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) depends on the implementation of the type of the source parameter. The expected behavior is that it invokes selector on each element of source to project it into a different form.