Not getting XFS Events[Windows Messages] (SERVICE_EVENTS, EXECUTE_EVENTS) in my application after registering using WFSRegister

Question

I have a C# console application where I'm using CEN XFS(Extensions for Financial Services) specification to get ATM information. I'm using PInvoke to communicate with msxfs.dll. I'm registering to listen for SERVICE_EVENTS, USER_EVENTS, SYSTEM_EVENTS, EXECUTE_EVENTS invoking method

HRESULT WFSRegister(hService, dwEventClass, hWndReg)

In my code I create an instance of WNDCLASSEX assign a method that will process Windows Messages to its lpfnWndProc field; then I register a class using RegisterClassEx([In] ref WNDCLASSEX lpWndClass) and create a Window calling CreateWindowEx.

In my code later I execute WFS_CMD_CDM_SET_CASH_UNIT_INFO command and it returns WFS_SUCCESS, base on the documentation this command will generate WFS_SRVE_CDM_CASHUNITINFOCHANGED service event but I'm not getting any window message with value 304 (#define WFS_SRVE_CDM_CASHUNITINFOCHANGED (CDM_SERVICE_OFFSET + 4)) or any message related with XFS Events.

It is possible that the msxfs.dll or Service Providers I'm using does not have events notification implemented. Am I missing something that prevents me to get windows messages?

Answer 1

In order to receive window messages you must have a separate thread which is running the equivalent of the standard Windows message pump:

    MSG     msg = { 0 };
    BOOL    bRet;

    while ((bRet = GetMessage(&msg, hWnd, 0, 0)) != 0) { 
    if (bRet == -1) {
        // handle the error and possibly exit
    } else {
        TranslateMessage(&msg); 
        DispatchMessage(&msg); 
    }
}

Furthermore, it is not necessary to do all this via P/Invoke — you can do it in C# natively by creating a hidden form and overriding WndProc. You still have to create and run that form on a separate thread, and to use Invoke when you need to close it.

Note that for the form to be fully hidden in addition to setting the FormBorderStyle to None, ShowIcon to false, ShownInTaskbar to false, Visible to false, and WindowState to Minimized, you should also change the Extended Window Styles to include WS_EX_TOOLWINDOW during form creation:

protected override CreateParams CreateParams {
    get {
        var Params = base.CreateParams;
        Params.ExStyle |= WS_EX_TOOLWINDOW;
        return Params;
    }
}

Finally, note that the WFSRESULT pointer which you get via lParam in WndProc when you receive an event needs to be freed using WFSFreeResult(). Inspecting the Buffer member of the WFSRESULT structure and marshaling any nested structures returned in it needs to be completed before calling WFSFreeResult() or the data will be invalid.