Enumerable::ToLookup<TSource, TKey, TElement> Method (IEnumerable<TSource>^, Func<TSource, TKey>^, Func<TSource, TElement>^)
Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to specified key selector and element selector functions.
Assembly: System.Core (in System.Core.dll)
public: generic<typename TSource, typename TKey, typename TElement> [ExtensionAttribute] static ILookup<TKey, TElement>^ ToLookup( IEnumerable<TSource>^ source, Func<TSource, TKey>^ keySelector, Func<TSource, TElement>^ elementSelector )
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.
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>^) 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.
The default equality comparer Default is used to compare keys.
The following code example demonstrates how to use ToLookup<TSource, TKey, TElement>(IEnumerable<TSource>^, Func<TSource, TKey>^, Func<TSource, TElement>^) to create a Lookup<TKey, TElement> by using a key selector function and an element selector function.
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