MissingMethodException on CoInitializedEx

Hi, I created a class. In the class, I created a method. In the method, I am trying to initialize the COM by calling ::CoInitializeEx(NULL,COINIT_APARTMENTTHREADED); and then I created a test problem on the PocketPC platform to test the class. When I build the project, I didn''t get any compilation error, but at run time, when the method that has the CoInitializedEx is called, it gives me a MissingMethodException. I think maybe it can not find the dll it needs. Am I right? Anyone has any idea? Thanks a lot. Ricky