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

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Creates a Lookup<(Of <(TKey, TElement>)>) from an IEnumerable<(Of <(T>)>) according to a specified key selector function and key comparer.

Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)


<ExtensionAttribute> _
Public Shared Function ToLookup(Of TSource, TKey) ( _
    source As IEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey), _
    comparer As IEqualityComparer(Of TKey) _
) As ILookup(Of TKey, TSource)
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.


  • keySelector
    Type: System..::.Func<(Of <(TSource, TKey>)>)
    A function to extract a key from each element.

Return Value

Type: System.Linq..::.ILookup<(Of <(TKey, TSource>)>)
A Lookup<(Of <(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<(Of <(TSource>)>). When you use instance method syntax to call this method, omit the first parameter.


Exception Condition

source or keySelector is nullNothingnullptra null reference (Nothing in Visual Basic).


The ToLookup<(Of <(TSource, TKey>)>)(IEnumerable<(Of <(TSource>)>), Func<(Of <(TSource, TKey>)>), IEqualityComparer<(Of <(TKey>)>)) method returns a Lookup<(Of <(TKey, TElement>)>), a one-to-many dictionary that maps keys to collections of values. A Lookup<(Of <(TKey, TElement>)>) is different to a Dictionary<(Of <(TKey, TValue>)>), which performs a one-to-one mapping of keys to single values.

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

Version Information

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0


Windows Phone

See Also


Enumerable Class

ToLookup Overload

System.Linq Namespace