Enumerable::ToLookup<TSource, TKey> Method (IEnumerable<TSource>^, Func<TSource, TKey>^, IEqualityComparer<TKey>^)
Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function and key comparer.
Assembly: System.Core (in System.Core.dll)
public: generic<typename TSource, typename TKey> [ExtensionAttribute] static ILookup<TKey, TSource>^ ToLookup( IEnumerable<TSource>^ source, Func<TSource, TKey>^ keySelector, IEqualityComparer<TKey>^ comparer )
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.
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<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 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