Get type parameters from type implementing IDictionary<,>

I would like to write a function to get the type parameters from types implementing IDictionary. What I have so far, and what is discussed in most SO questions is:

public static Type[] GetParameters(Type dicType) {

    if (typeof(IDictionary).IsAssignableFrom(dicType)) {
        return dicType.GetGenericArguments();
    }
    else {
        return null;
    }
}

However this fails with the following:

public class MyClass : Dictionary<string, int> { }

Type[] typeParams = GetParameters(typeof(MyClass));
Console.WriteLine(String.Join(", ", (object[]) typeParams));

which would print nothing (empty array). This is even worse with the following:

public class MyOtherClass<T, U> : Dictionary<string, string> { }

Type[] typeParams = GetParameters(typeof(MyOtherClass<int, int>));
Console.WriteLine(String.Join(", ", (object[]) typeParams));

as the output is completely wrong (System.Int32, System.Int32).

How can I get these parameters for any type that inherits from IDictionary ?