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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 Parameters


The type of the elements of source.


The type of the key returned by keySelector.


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.

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).


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.

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

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.