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.NET Framework Class Library
EnumerableToLookupTSource, TKey Method (IEnumerableTSource, FuncTSource, TKey)

Creates a LookupTKey, TElement from an IEnumerableT according to a specified key selector function.

Namespace:   System.Linq
Assemblies:   System.Linq (in System.Linq.dll)
  System.Core (in System.Core.dll)
<[%$TOPIC/bb549073_en-us_VS_110_3_0_0_0_0%]> _
Public Shared Function ToLookup(Of TSource, TKey) ( _
	source As [%$TOPIC/bb549073_en-us_VS_110_3_0_0_0_1%](Of TSource), _
	keySelector As [%$TOPIC/bb549073_en-us_VS_110_3_0_0_0_2%](Of TSource, TKey) _
) As [%$TOPIC/bb549073_en-us_VS_110_3_0_0_0_3%](Of TKey, TSource)
public static [%$TOPIC/bb549073_en-us_VS_110_3_0_1_0_0%]<TKey, TSource> ToLookup<TSource, TKey>(
	this [%$TOPIC/bb549073_en-us_VS_110_3_0_1_0_1%]<TSource> source,
	[%$TOPIC/bb549073_en-us_VS_110_3_0_1_0_2%]<TSource, TKey> keySelector
generic<typename TSource, typename TKey>
static [%$TOPIC/bb549073_en-us_VS_110_3_0_2_0_1%]<TKey, TSource>^ ToLookup(
	[%$TOPIC/bb549073_en-us_VS_110_3_0_2_0_2%]<TSource>^ source, 
	[%$TOPIC/bb549073_en-us_VS_110_3_0_2_0_3%]<TSource, TKey>^ keySelector
static member ToLookup : 
        source:[%$TOPIC/bb549073_en-us_VS_110_3_0_3_0_0%]<'TSource> * 
        keySelector:[%$TOPIC/bb549073_en-us_VS_110_3_0_3_0_1%]<'TSource, 'TKey> -> [%$TOPIC/bb549073_en-us_VS_110_3_0_3_0_2%]<'TKey, 'TSource> 

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


Type: System.Collections.GenericIEnumerableTSource

The IEnumerableT to create a LookupTKey, TElement from.

Type: SystemFuncTSource, TKey

A function to extract a key from each element.

Return Value

Type: System.LinqILookupTKey, TSource
A LookupTKey, TElement that contains keys and values.

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 keySelector is .


The ToLookupTSource, TKey(IEnumerableTSource, FuncTSource, TKey) method returns a LookupTKey, TElement, a one-to-many dictionary that maps keys to collections of values. A LookupTKey, TElement differs from a DictionaryTKey, TValue, which performs a one-to-one mapping of keys to single values.

The default equality comparer Default is used to compare keys.

Version Information

.NET Framework

Supported in: 4.6, 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

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: 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.