Question

If it exists, what is the C# equivalent of the following Java code:

new HashMap<Class<? extends BaseClass>, Integer>();

I currently use new Dictionary<Type, int>(), which is more like
new HashMap<Class<?>, Integer>() which is obviously not the same.

(Ignore the differences between HashMap and Dictionary)

Edit: To clarify, I am not trying to define a new class, simply create an instance of HashMap/Dictionary.

Answer 1

There is no equivalent of the Java wildcard in C#. In Java, the type for types is Class<T> where T is the class itself. The equivalent in C# is the type Type, which is not generic. So it seems that the best you can do is to have, as you said, a Dictionary<Type, int>, and if it’s encapsulated in a class you can restrict what you put in the dictionary in the code (so it will just be a runtime check):

private Dictionary<Type, int> myDictionary = new Dictionary<Type, int>();
public void Add(Type type, int number) {
   if (!typeof(BaseClass).IsAssignableFrom(type)) throw new Exception();
   myDictionary.Add(type, number);
}

You can even implement your own IDictionary with that logic.

UPDATE

Another runtime trick I can think of is to use a wrapper class for your types:

public class TypeWrapper<T>
{
    public Type Type { get; private set; }
    public TypeWrapper(Type t)
    {
        if (!typeof(T).IsAssignableFrom(t)) throw new Exception();
        Type = t;
    }
    public static implicit operator TypeWrapper<T>(Type t) {
        return new TypeWrapper<T>(t);
    }
}

(Also implement Equals and GetHashCode, just delegate to Type.)

And then your dictionary becomes:

var d = new Dictionary<TypeWrapper<BaseClass>, int>();
d.Add(typeof(BaseClass), 2);
d.Add(typeof(Child), 3);