What's the C# syntax for getting a pointer to a struct, so you can pass it
as an untyped parameter like lParam?

And please nobody tell me that I can alias PostMessage and type wParam as
ref NMHDR because I already know that. There is a way of getting a pointer
to a struct or a variable and I want to know what it is so that I can look
it up and read all about the surrounding issues and then apply this
knowledge to other things like the incorrect declaration of POLECHAR as ref
ushort in the mshtml PIA.