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.NET Framework Class Library
EnumerableSelectTSource, TResult Method (IEnumerableTSource, FuncTSource, Int32, TResult)

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

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)
<[%$TOPIC/bb534869_en-us_VS_110_1_0_0_0_0%]> _
Public Shared Function Select(Of TSource, TResult) ( _
	source As [%$TOPIC/bb534869_en-us_VS_110_1_0_0_0_1%](Of TSource), _
	selector As [%$TOPIC/bb534869_en-us_VS_110_1_0_0_0_2%](Of TSource, [%$TOPIC/bb534869_en-us_VS_110_1_0_0_0_3%], TResult) _
) As [%$TOPIC/bb534869_en-us_VS_110_1_0_0_0_4%](Of TResult)
public static [%$TOPIC/bb534869_en-us_VS_110_1_0_1_0_0%]<TResult> Select<TSource, TResult>(
	this [%$TOPIC/bb534869_en-us_VS_110_1_0_1_0_1%]<TSource> source,
	[%$TOPIC/bb534869_en-us_VS_110_1_0_1_0_2%]<TSource, [%$TOPIC/bb534869_en-us_VS_110_1_0_1_0_3%], TResult> selector
generic<typename TSource, typename TResult>
static [%$TOPIC/bb534869_en-us_VS_110_1_0_2_0_1%]<TResult>^ Select(
	[%$TOPIC/bb534869_en-us_VS_110_1_0_2_0_2%]<TSource>^ source, 
	[%$TOPIC/bb534869_en-us_VS_110_1_0_2_0_3%]<TSource, [%$TOPIC/bb534869_en-us_VS_110_1_0_2_0_4%], TResult>^ selector
static member Select : 
        source:[%$TOPIC/bb534869_en-us_VS_110_1_0_3_0_0%]<'TSource> * 
        selector:[%$TOPIC/bb534869_en-us_VS_110_1_0_3_0_1%]<'TSource, [%$TOPIC/bb534869_en-us_VS_110_1_0_3_0_2%], 'TResult> -> [%$TOPIC/bb534869_en-us_VS_110_1_0_3_0_3%]<'TResult> 

Type Parameters


The type of the elements of source.


The type of the value returned by selector.


Type: System.Collections.GenericIEnumerableTSource

A sequence of values to invoke a transform function on.

Type: SystemFuncTSource, Int32, TResult

A transform function to apply to each source element; the second parameter of the function represents the index of the source element.

Return Value

Type: System.Collections.GenericIEnumerableTResult
An IEnumerableT whose elements are the result of invoking the transform function on each element of source.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerableTSource. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

source or selector is .


This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The first argument to selector represents the element to process. The second argument to selector represents the zero-based index of that element in the source sequence. This can be useful if the elements are in a known order and you want to do something with an element at a particular index, for example. It can also be useful if you want to retrieve the index of one or more elements.

This projection method requires the transform function, selector, to produce one value for each value in the source sequence, source. If selector returns a value that is itself a collection, it is up to the consumer to traverse the subsequences manually. In such a situation, it might be better for your query to return a single coalesced sequence of values. To achieve this, use the SelectMany method instead of Select. Although SelectMany works similarly to Select, it differs in that the transform function returns a collection that is then expanded by SelectMany before it is returned.


The following code example demonstrates how to use SelectTSource, TResult(IEnumerableTSource, FuncTSource, Int32, TResult) to project over a sequence of values and use the index of each element.

        ' Create an array of strings. 
        Dim fruits() As String = _
            {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

        ' Project each item in the array to an anonymous type 
        ' that stores the item's index in the array and 
        ' a substring of each item whose length is equal 
        ' to the index position in the original array. 
        Dim query = _
            fruits.Select(Function(fruit, index) _
                              New With {index, .Str = fruit.Substring(0, index)})

        Dim output As New System.Text.StringBuilder
        For Each obj In query

        ' Display the output.

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

            var query =
                fruits.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}
Version Information

.NET Framework

Supported in: 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.