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Enumerable.ToLookup(Of TSource, TKey) Method (IEnumerable(Of TSource), Func(Of TSource, TKey), IEqualityComparer(Of TKey))

Creates a Lookup(Of TKey, TElement) from an IEnumerable(Of T) according to a specified key selector function and key comparer.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)
'Declaration
<ExtensionAttribute> _
Public Shared Function ToLookup(Of TSource, TKey) ( _
	source As IEnumerable(Of TSource), _
	keySelector As Func(Of TSource, TKey), _
	comparer As IEqualityComparer(Of TKey) _
) As ILookup(Of TKey, TSource)

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of TSource)

The IEnumerable(Of T) to create a Lookup(Of TKey, TElement) from.

keySelector
Type: System.Func(Of TSource, TKey)

A function to extract a key from each element.

comparer
Type: System.Collections.Generic.IEqualityComparer(Of TKey)

An IEqualityComparer(Of T) to compare keys.

Return Value

Type: System.Linq.ILookup(Of TKey, TSource)
A Lookup(Of TKey, 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 IEnumerable(Of TSource). 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).
ExceptionCondition
ArgumentNullException

source or keySelector is Nothing.

The ToLookup(Of TSource, TKey)(IEnumerable(Of TSource), Func(Of TSource, TKey), IEqualityComparer(Of TKey)) method returns a Lookup(Of TKey, TElement), a one-to-many dictionary that maps keys to collections of values. A Lookup(Of TKey, TElement) is different to a Dictionary(Of TKey, TValue), which performs a one-to-one mapping of keys to single values.

If comparer is Nothing, the default equality comparer Default is used to compare keys.

.NET Framework

Supported in: 4.5.1, 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 8, Silverlight 8.1

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.

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