Was this page helpful?
Your feedback about this content is important. Let us know what you think.
Additional feedback?
1500 characters remaining
MSDN Library

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

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

Namespace:  System.Linq
Assemblies:   System.Linq (in System.Linq.dll)
  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) _
) 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.

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)) method returns a Lookup(Of TKey, TElement), a one-to-many dictionary that maps keys to collections of values. A Lookup(Of TKey, TElement) differs from a Dictionary(Of TKey, TValue), which performs a one-to-one mapping of keys to single values.

The default equality comparer Default is used to compare keys.

.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
Show:
© 2015 Microsoft