Restricting 'Type' parameters in a method/constructor

Question

I'd like to pass a Type parameter into a constructor. This constructor belongs to an attribute.

That's simple. But how do I constrain this Type parameter to sub classes of a particular class only?

So I have a parent class ParentClass and two child classes MyChildClass : ParentClass and MyOtherChildClass : ParentClass.

My attribute looks like this:

public class AssociatedTypeAttribute : Attribute
{
    private readonly Type _associatedType;

    public Type AssociatedType => _associatedType;

    public AssociatedTypeAttribute(Type associatedType)
    {
        if (!associatedType.IsSubclassOf(typeof(ParentClass)))
            throw new ArgumentException($"Specified type must be a {nameof(Parentclass)}, {associatedType.Name} is not.");

        _associatedType = associatedType;
    }
}

This works, and at runtime it'll throw an exception if the type isn't a ParentClass - but runtime is too late.

Is it possible to add some sort of constraint? Can I use generics here or am I right in saying generics are out of bounds since it's the constructor of an attribute?

Note usage:

public enum MyEnum
{
   [AssociatedType(typeof(MyChildClass))]
   MyEnumValue,
   [AssociatedType(typeof(MyOtherChildClass))]
   MyOtherEnumValue
}

Answer 1

You can't do this with Type and you can't use generics because it is not allowed to extend Attribute with a generic class.

The best solution you have is a runtime check and simply ignoring the attribute if the target doesn't match the intended one.