Enumerable.ToLookup<(Of <(TSource, TKey>)>) Method (IEnumerable<(Of <(TSource>)>), Func<(Of <(TSource, TKey>)>), IEqualityComparer<(Of <(TKey>)>))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
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
<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<(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.
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.
Exceptions
| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector is nullNothingnullptra null reference (Nothing in Visual Basic). |
Remarks
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 nullNothingnullptra null reference (Nothing in Visual Basic), the default equality comparer Default is used to compare keys.
Version Information
Windows Phone OS
Supported in: 8.1, 8.0, 7.1, 7.0
Platforms
Windows Phone