Enumerable.ToLookup(Of TSource, TKey, TElement) Method (IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement), 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, a comparer and an element selector function.
Assembly: System.Core (in System.Core.dll)
'Declaration <ExtensionAttribute> _ Public Shared Function ToLookup(Of TSource, TKey, TElement) ( _ source As IEnumerable(Of TSource), _ keySelector As Func(Of TSource, TKey), _ elementSelector As Func(Of TSource, TElement), _ comparer As IEqualityComparer(Of TKey) _ ) As ILookup(Of TKey, TElement)
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by keySelector.
- TElement
The type of the value returned by elementSelector.
Parameters
- source
- Type: System.Collections.Generic.IEnumerable(Of TSource)
The IEnumerable(Of T) to create a Lookup(Of TKey, TElement) from.
- keySelector
- Type: System.Func(Of TSource, TKey)
A function to extract a key from each element.
- elementSelector
- Type: System.Func(Of TSource, TElement)
A transform function to produce a result element value from each element.
- comparer
- Type: System.Collections.Generic.IEqualityComparer(Of TKey)
An IEqualityComparer(Of T) to compare keys.
Return Value
Type: System.Linq.ILookup(Of TKey, TElement)A Lookup(Of 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(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector or elementSelector is Nothing. |
The ToLookup(Of TSource, TKey, TElement)(IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement), 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) differs from a Dictionary(Of TKey, TValue), which performs a one-to-one mapping of keys to single values.
If comparer is Nothing, the default equality comparer Default is used to compare keys.