What exactly do you want to do? Is it for use inside C #? Or from the outside (COM)?
, , , WebBrowser ObjectForScripting.
# 3.0 :
public static void Bar(this Foo foo, int someArg) {...}
....
Foo foo = ...
foo.Bar(123);
. # 4.0 dynamic CLR/DLR, IDynamicObject. .