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)
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(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.
- elementSelector
- Type: System.Func(TSource, TElement)
A transform function to produce a result element value from each element.
- comparer
- 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.| Exception | Condition |
|---|---|
| ArgumentNullException | 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.
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