Enumerable.ToLookup<TSource, TKey> Method (IEnumerable<TSource>, Func<TSource, TKey>, IEqualityComparer<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 and key comparer.
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), _
comparer As IEqualityComparer(Of TKey) _
) As ILookup(Of TKey, TSource)
public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey> comparer
)
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.
- comparer
Type: System.Collections.Generic.IEqualityComparer<TKey>
An IEqualityComparer<T> to compare keys.
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>, IEqualityComparer<TKey>) method returns a Lookup<TKey, TElement>, a one-to-many dictionary that maps keys to collections of values. A Lookup<TKey, TElement> is different to a Dictionary<TKey, TValue>, which performs a one-to-one mapping of keys to single values.
If comparer is nulla null reference (Nothing in Visual Basic), 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.