I'm new to XS programming. I've tried to declare the 'uType' input field to the function as 'UINT' (since this is the way in C the function is declared):

int WINAPI MessageBox(
  __in_opt  HWND hWnd,
  __in_opt  LPCTSTR lpText,
  __in_opt  LPCTSTR lpCaption,
  __in      UINT uType
);
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My first impression from the error message was that I'm completely forbidden to use windows.h declared data types. Now I see that I can use them safely _inside_ a function. The problem only arises with the declaration and passing of args to the function (which is not a big deal: I want to pass only very simple input to the sub, rather use complicated types like NOTIFYICONDATA inside the sub).