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)
public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer )
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.
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.
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 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.
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.