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

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

Namespace:   System.Linq
Assemblies:   System.Linq (in System.Linq.dll)
  System.Core (in System.Core.dll)
<[%$TOPIC/bb549346_en-us_VS_110_3_0_0_0_0%]> _
Public Shared Function ToLookup(Of TSource, TKey) ( _
	source As [%$TOPIC/bb549346_en-us_VS_110_3_0_0_0_1%](Of TSource), _
	keySelector As [%$TOPIC/bb549346_en-us_VS_110_3_0_0_0_2%](Of TSource, TKey), _
	comparer As [%$TOPIC/bb549346_en-us_VS_110_3_0_0_0_3%](Of TKey) _
) As [%$TOPIC/bb549346_en-us_VS_110_3_0_0_0_4%](Of TKey, TSource)
public static [%$TOPIC/bb549346_en-us_VS_110_3_0_1_0_0%]<TKey, TSource> ToLookup<TSource, TKey>(
	this [%$TOPIC/bb549346_en-us_VS_110_3_0_1_0_1%]<TSource> source,
	[%$TOPIC/bb549346_en-us_VS_110_3_0_1_0_2%]<TSource, TKey> keySelector,
	[%$TOPIC/bb549346_en-us_VS_110_3_0_1_0_3%]<TKey> comparer
generic<typename TSource, typename TKey>
static [%$TOPIC/bb549346_en-us_VS_110_3_0_2_0_1%]<TKey, TSource>^ ToLookup(
	[%$TOPIC/bb549346_en-us_VS_110_3_0_2_0_2%]<TSource>^ source, 
	[%$TOPIC/bb549346_en-us_VS_110_3_0_2_0_3%]<TSource, TKey>^ keySelector, 
	[%$TOPIC/bb549346_en-us_VS_110_3_0_2_0_4%]<TKey>^ comparer
static member ToLookup : 
        source:[%$TOPIC/bb549346_en-us_VS_110_3_0_3_0_0%]<'TSource> * 
        keySelector:[%$TOPIC/bb549346_en-us_VS_110_3_0_3_0_1%]<'TSource, 'TKey> * 
        comparer:[%$TOPIC/bb549346_en-us_VS_110_3_0_3_0_2%]<'TKey> -> [%$TOPIC/bb549346_en-us_VS_110_3_0_3_0_3%]<'TKey, 'TSource> 
JScript does not support generic types or methods.

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.

Type: System.Collections.GenericIEqualityComparerTKey

An IEqualityComparerT to compare keys.

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, IEqualityComparerTKey) method returns a LookupTKey, TElement, a one-to-many dictionary that maps keys to collections of values. A LookupTKey, TElement is different to a DictionaryTKey, TValue, which performs a one-to-one mapping of keys to single values.

If comparer is , 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

XNA Framework

Supported in: 3.0

.NET for Windows Phone apps

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

Portable Class Library

Supported in: Portable Class Library