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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)

static member ToLookup : 
        source:IEnumerable<'TSource> * 
        keySelector:Func<'TSource, 'TKey> * 
        comparer:IEqualityComparer<'TKey> -> ILookup<'TKey, 'TSource> 

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 a null reference (Nothing in Visual Basic).

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 a null reference (Nothing in Visual Basic), 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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