Ever since Maui, I've been torn by this question too.  The user is able to drag
whatever naked object they want right into the parameters of naked message
widgets.

I've tried putting type-checks in the tops of those public methods exposed on
the UI, but it would be nice if the invalid drops could be blocked in the first
place, and without all the extra code.

I've gotten incredible mileage out of Becks "Type Suggesting Parameter Name",
but it seems to be reaching its limits here.  Pretty soon, you find yourself
carefully coding method argument names in a way that will be friendly to Maui's
type-parsing logic.

Maybe MethodAnnotations will help..