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.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector,
	IEqualityComparer<TKey> comparer


Type: System.Collections.Generic.IEnumerable<TSource>

The IEnumerable<T> to create a Lookup<TKey, TElement> from.

Type: System.Func<TSource, TKey>

A function to extract a key from each element.

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


The type of the elements of source.


The type of the key returned by keySelector.

Exception Condition

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.

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
Return to top