C# - list of subclass Types

Question

I'd like to have a List of Class Types (not a list of Class instances) where each member of the List is a Subclass of MyClass.

For example, i can do this:

List<System.Type> myList;
myList.Add(typeof(mySubClass));

but i'd like to restrict the list to only accept subclasses of MyClass.

This is distinct from questions like this . Ideally i'd like to avoid linq, as it's currently unused in my project.

Answer 1

Servy is right in his comment , and Lee in his : it's much more preferable to compose than inherit . So this is a good option:

public class ListOfTypes<T>
{
    private List<Type> _types = new List<Type>();
    public void Add<U>() where U : T
    {
        _types.Add(typeof(U));
    }
}

Usage:

var x = new ListOfTypes<SuperClass>();
x.Add<MySubClass>()

Note that you can make this class implement an interface like IReadOnlyList<Type> if you want to give other code read access to the contained Type s without other code having to depend on this class.

But if you want to inherit anyway, you could create your own class that inherits from List , then add your own generic Add method like this:

public class ListOfTypes<T> : List<Type>
{
    public void Add<U>() where U : T
    {
        Add(typeof(U));
    }
}

Just be aware of what Lee said : with this second version you can still Add(typeof(Foo)) .

Answer 2

You should derive a list class from List and override the Add method to perform the type checking that you need. I'm not aware of a way in .NET to do that automatically.

Something like this could work:

public class SubTypeList : List<System.Type>
{
    public System.Type BaseType { get; set; }

    public SubTypeList()
        : this(typeof(System.Object))
    {
    }

    public SubTypeList(System.Type baseType)
    {
        BaseType = BaseType;
    }

    public new void Add(System.Type item)
    {
        if (item.IsSubclassOf(BaseType) == true)
        {
            base.Add(item);
        }
        else
        {
            // handle error condition where it's not a subtype... perhaps throw an exception if
        }
    }
}

You would need to update the other methods that add/update items to the list (index setter, AddRange, Insert, etc)