.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)
Syntax
<[%$TOPIC/bb549346_en-us_VS_110_3_0_0_0_0%]> _
Public Shared Function ToLookup(Of TSource, TKey) ( _
	source As [%$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,
	[%$TOPIC/bb549346_en-us_VS_110_3_0_1_0_3%]<TKey> comparer
)
[[%$TOPIC/bb549346_en-us_VS_110_3_0_2_0_0%]]
public:
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

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters

source
Type: System.Collections.GenericIEnumerableTSource

The IEnumerableT to create a LookupTKey, TElement from.

keySelector
Type: SystemFuncTSource, TKey

A function to extract a key from each element.

comparer
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).
Exceptions
ExceptionCondition
ArgumentNullException

source or keySelector is .

Remarks

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