Enumerable.ToLookup(TSource, TKey) Method (IEnumerable(TSource), Func(TSource, TKey))
Creates a Lookup(TKey, TElement) from an IEnumerable(T) according to a specified key selector function.
Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)
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.
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.| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector is a null reference (Nothing in Visual Basic). |
The ToLookup(TSource, TKey)(IEnumerable(TSource), Func(TSource, 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.
The default equality comparer Default is used to compare keys.
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