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Enumerable.ToLookup<TSource, TKey> Method (IEnumerable<TSource>, Func<TSource, TKey>)

Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to a specified key selector function.

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
)

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.

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.

ExceptionCondition
ArgumentNullException

source or keySelector is null.

The ToLookup<TSource, TKey>(IEnumerable<TSource>, Func<TSource, 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.

The default equality comparer Default is used to compare keys.

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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