MyFancyThingAddinControl.cs

[ControlAddInExport("My Fancy Thing Addin Control")]
public class MyFancyThingAddinControl : WinFormsControlAddInBase
{
    private MyFancyThing _myFancyThing;

    [ApplicationVisible]
    // no need for null checks later
    public event MethodInvoker ControlAddInReady = delegate { };

    protected override Control CreateControl()
    {
        _myFancyThing = new MyFancyThing();
        _myFancyThing.ParentChanged += (sender, e) => ControlAddInReady();
        return _myFancyThing;
    }
}