Windows Common Item Dialog: ctypes + COM access violation

Question

I am trying to use the ctypes module to make calls in the Windows Common Dialog API . The following example is based on the steps described in the documentation.

The resulting macro will open the Open File dialog box, as expected. In both cases, the vtable pointer offsets are the same, but only the Python version causes an access violation.

Can anyone shed some light on this?

[We apologize for not adding more hyperlinks if necessary, but as a new user I’m limited to two at a time.]

Background : I am building a lightweight module that provides its own save / open file dialog for use in Python scripts. So far, I have not been able to find an implementation in pure Python. Those that exist rely on user interface tools such as Tkinter or wxPython.

Answer 1

Here is the solution:

COM methods accept an additional parameter: a pointer to 'this'. This is implied when you call a method from C ++, in C (and in ctypes) you must provide it yourself.

Change the line

 func_proto = ctypes.WINFUNCTYPE(HRESULT, HWND) 

in

 func_proto = ctypes.WINFUNCTYPE(HRESULT, c_long, HWND) 

and this line

 show(0) 

in

 show(ptr, 0) 

and your code will work.