Enumerable.ToLookup(Of TSource, TKey) Method (IEnumerable(Of TSource), Func(Of TSource, 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.
Assembly: System.Core (in System.Core.dll)
'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)
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.
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.| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector is Nothing. |
The ToLookup(Of TSource, TKey)(IEnumerable(Of TSource), Func(Of TSource, 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) differs from a Dictionary(Of TKey, TValue), which performs a one-to-one mapping of keys to single values.
The default equality comparer Default is used to compare keys.