Interface methods lack pure specifiers. This might be OK for some development but once you try to inherit your handler class from public winrt::implements<AsyncCallback, IRtwqAsyncCallback> you are in trouble! 1>Foo.obj : error LNK2001: unresolved external symbol “public: virtual long __cdecl IRtwqAsyncCallback::GetParameters(unsigned long *,unsigned long *)” (?GetParameters@IRtwqAsyncCallback@@UEAAJPEAK0@Z) 1>Foo.obj : error LNK2001: unresolved external symbol “public: virtual…