Enumerable.ToLookup(Of TSource, TKey, TElement) Method (IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement), IEqualityComparer(Of TKey))
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)
<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)
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.
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.
| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector or elementSelector is null. |
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 null, the default equality comparer Default is used to compare keys.
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1