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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
Assemblies:   System.Linq (in System.Linq.dll)
  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

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.

comparer
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).

ExceptionCondition
ArgumentNullException

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: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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