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)
[ExtensionAttribute] public: generic<typename TSource, typename TKey> static ILookup<TKey, TSource>^ ToLookup( IEnumerable<TSource>^ source, Func<TSource, TKey>^ keySelector, IEqualityComparer<TKey>^ comparer )
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<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.
Usage Note
In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector is nullptr. |
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 nullptr, the default equality comparer Default is used to compare keys.
Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.