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 nullptr. |
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 nullptr, the default equality comparer Default is used to compare keys.
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