Enumerable.ToLookup(Of TSource, TKey) Method (IEnumerable(Of TSource), Func(Of TSource, TKey))
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)
<ExtensionAttribute> Public Shared Function ToLookup(Of TSource, TKey) ( source As IEnumerable(Of TSource), keySelector As Func(Of TSource, TKey) ) As ILookup(Of TKey, TSource)
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.
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by keySelector.
| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector is null. |
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.
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1