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

.NET Framework (current version)

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)

Syntax

C++

<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)

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.

Type Parameters

TSource: The type of the elements of source.

TKey: The type of the key returned by keySelector.

Exceptions

Exception	Condition
ArgumentNullException	source or keySelector is null.

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 null, the default equality comparer Default is used to compare keys.

Version Information

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1