如何接口 IGrouping<out tkey, out telement> 产生多个值？</out>

Question

I wonder what point I overlook. IGrouping<out TKey, out TElement> is used by LINQ GroupBy . Further, the same logic is applied to ILookup , IIRC. Accordingly, they can return multiple values subject to the key specified. However, instead, I would expect the following signature to be done so.

interface IGrouping<out TKey, out IEnumerable<TElement>>
                                  ^^^^^^^^^^^^^

Originally, it implements IEnumerable<TElement> and IEnumerable for backward compatibility. Does this accomplish the same task? If yes, how? I don't understand.

public interface ILookup<TKey,TElement> : IEnumerable<IGrouping<TKey,TElement>>

Does the followed mean ILookup used by Join/GroupJoin has multiple keys with the association of multiple values? Could you exemplify? I've looked the question , but don't understand the difference, except that their lazy evaluation vs immediate evaluation semantics.

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Not to do complicate matter further , but the output may help for everyone.

Answer 1

If you look at the declaration of IGrouping ,

public interface IGrouping<out TKey,out TElement> : System.Collections.Generic.IEnumerable<out TElement>

You can see that it says " IGrouping<TKey, TElement> is a kind of IEnumerable<TElement> ".

It's a special kind of IEnumerable<TElement> . Special in what way? It has a Key associated.

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And no, this is not valid C# syntax:

interface IGrouping<out TKey, out IEnumerable<TElement>>

Remember that this is an interface declaration , and in the <> , it is declaring the type parameters, which are identifiers, for the interface. IEnumerable<TElement> is not a valid identifier. You also seem to want to refer to the existing interface System.Collections.Generic.IEnumerable<T> , but it makes no sense to put it in this place in the declaration, the same way that it wouldn't make sense to write interface Foo<List<T>> as an interface declaration in Java.

If you mean:

interface IGrouping<out TKey, out TElement> where TElement : IEnumerable

that could work, and there are many ways in which you can design this IGrouping interface. The following way probably makes sense to you the most?

interface IMyGrouping<TKey, TElement> {
    TKey Key { get; }
    IEnumerable<TElement> Elements { get; }
}

But what's really great about .NET framework's design is that it implements an existing interface , and that allows it to be used as an IEnumerable<TElement> directly, which is a very powerful thing. If your class have the ability that the interface promises, implement that interface. See also .

Does the followed mean ILookup used by GroupBy has multiple keys with the association of multiple values?

Yes. Since ILookup implements IEnumerable<IGrouping> , and IGrouping implements IEnumerable , ILookup is kind of like a IEnumerable<IEnumerable> (forgive me using the generics notation very loosely here). So an ILookup an enumerable of enumerables, ie like a 2D array. The difference is, that each inner "array" has a Key , since the inner "arrays" are actually IGrouping s.