Custom Activity with Type Argument WF 4 (C# Workflow Foundation)

Question

First time question here.. wow I can't post images.. oh well.

I am trying to implement a custom activity for a workflow that takes a type argument <T>. It looks like this

public class AddDataColumn<T> : CodeActivity
{
    [Category("DefaultValue")]
    public InArgument<T> DefaultValue
    {
        get;
        set;
    }

However, when I attempt to use it in the workflow I am prompted with a window asking me for a type argument

https://i.stack.imgur.com/SERPJ.png

I wish to implement something like this so that I can change the argument type at design time:

https://i.stack.imgur.com/ABsCW.png

Is there any way to do this? Any help is appreciated, thank you.