there appears to be some issue when a type parameter is a ref parameter to a generic type's method. test case below. import System.Collections.Generic class Foo: a as string dict = Dictionary [of string, Foo]() dict["cow"] = Foo() bar as Foo dict.TryGetValue("cow", bar) compilation results in: "boo.sample(10,17): BCE0017: The best overload for the method 'System.Collections.Generic.Dictionary`2[[System.String], [Foo]].TryGetValue(string, ref System.Collections.Generic.Dictionary`2.TValue)' is not compatible with the argument list '(string, Foo)'."