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
- source
Type: System.Collections.Generic.IEnumerable<TSource>
The IEnumerable<T> to create a Lookup<TKey, TElement> from.
- 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.