• Article

Enumerable.ToLookup<TSource, TKey> Method (IEnumerable<TSource>, Func<TSource, TKey>)

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

Syntax

'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)

public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TKey
    The type of the key returned by keySelector.

Parameters

  • keySelector
    Type: System.Func<TSource, TKey>
    A function to extract a key from each element.

Return Value

Type: System.Linq.ILookup<TKey, TSource>
A Lookup<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<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source or keySelector is nulla null reference (Nothing in Visual Basic).

Remarks

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

The default equality comparer Default is used to compare keys.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.