Enumerable.ToLookup<'TSource, 'TKey, 'TElement> Method (IEnumerable<'TSource>, Func<'TSource, 'TKey>, Func<'TSource, 'TElement>, IEqualityComparer<'TKey>)
Creates a Lookup<'TKey, 'TElement> from an IEnumerable<'T> according to a specified key selector function, a comparer and an element selector function.
Assembly: System.Core (in System.Core.dll)
static member ToLookup<'TSource, 'TKey, 'TElement> : source:IEnumerable<'TSource> * keySelector:Func<'TSource, 'TKey> * elementSelector:Func<'TSource, 'TElement> * comparer:IEqualityComparer<'TKey> -> ILookup<'TKey, 'TElement>
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.
- elementSelector
-
Type:
System.Func<'TSource, 'TElement>
A transform function to produce a result element value from each element.
- comparer
-
Type:
System.Collections.Generic.IEqualityComparer<'TKey>
An IEqualityComparer<'T> to compare keys.
Return Value
Type: System.Linq.ILookup<'TKey, 'TElement>A Lookup<'TKey, 'TElement> that contains values of type TElement selected from the input sequence.
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by keySelector.
- TElement
The type of the value returned by elementSelector.
| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector or elementSelector is null. |
The ToLookup<'TSource, 'TKey, 'TElement>(IEnumerable<'TSource>, Func<'TSource, 'TKey>, Func<'TSource, 'TElement>, IEqualityComparer<'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.
If comparer is null, 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