语法：派生类型，嵌套泛型

Question

I want to have a dictionary that maps from one generic type K<T> to another generic type V<T> . Note that type T should be the same in both cases. So I thought I would create a derived class from Dictionary<K,V> with one more type argument, but I cannot find the right syntax.

This is what I tried:

1.

class MyDictionary<T,K,V> : Dictionary<K<T>,V<T>> { ... }
// syntax error: The type parameter K cannot be used with type arguments.
//               (same with V)

2.

class MyDictionary<T,K<T>,V<T>> : Dictionary<K,V> { ... }
// many syntax errors: "," expected; "{" expected; ...

3.

class MyDictionary<T,K,V> : Dictionary<K,V> { ... }
// works, but does not make the connection between K and T and between V and T

Maybe I can use some where clause?

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Some examples for better understanding:

MyDictionary<int,List,HashSet> should essentially be Dictionary<List<int>,HashSet<int>> .

MyDictionary<float,Queue,Stack> should essentially be Dictionary<Queue<float>, Stack<float>> .

Answer 1

Use generic type constraints ; based on your examples you might want to settle on some base class/interface for the two. For example, the following would work for both of your cases:

class MyDictionary<T,K,V> : Dictionary<K,V>
    where K : IEnumerable<T>
    where V : IEnumerable<T>

If that's not viable, ie you need to be able to store mappings between, say, Action<String> and IEqualityComparer<String> , you won't be able to do that directly; you can at best settle for checking generic type parameters at runtime (eg in your the constructor for your class).

Answer 2

Seems you need something like this:

class ClassK<T> { }
class ClassV<T> { }
class MyComplexDictionary<T> : Dictionary<K<T>, V<T>> { }

It will prevent you from adding incorrectly typed instances of K<> and V in compile time.

    var a = new MyComplexDictionary<string>();
    a.Add(new ClassK<string>(), new ClassV<string>()); //works
    a.Add(new ClassK<int>(), new ClassV<string>()); // compilation error