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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)

JScript does not support generic types or methods.

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.


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.Func(TSource, TElement)

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

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 or elementSelector is a null reference (Nothing in Visual Basic).

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 a null reference (Nothing in Visual Basic), the default equality comparer Default is used to compare keys.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0