ToLookup(TSource, TKey, TElement) Method (IEnumerable(TSource), Func(TSource, TKey), Func(TSource, TElement), IEqualityComparer(TKey))

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.

Namespace:   System.Linq
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: System.Collections.Generic.IEnumerable<TSource>

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

Type: System.Func<TSourceTKey>

A function to extract a key from each element.

Type: System.Func<TSourceTElement>

A transform function to produce a result element value from each element.

Type: System.Collections.Generic.IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys.

Return Value

Type: System.Linq.ILookup<TKeyTElement>

A Lookup<TKey, TElement> that contains values of type TElement selected from the input sequence.

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the value returned by elementSelector.

Exception Condition

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.

Universal Windows Platform
Available since 4.5
.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
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