I have dialog created using CreateDialogParam. It has only a single ListView child control. In the dialog WM_INITDIALOG message handler, I subclass the ListView to customize header redrawing.

Now I want to prevent the user from resizing the ListView column (header) and, to do this, I just need to handle the HDN_BEGINTRACKA notification message in the ListView's WndProc, like below:

case WM_NOTIFY:
    {
        if ((((LPNMHDR)lParam)->code == HDN_BEGINTRACKA)
            || (((LPNMHDR)lParam)->code == HDN_BEGINTRACKW))
            return TRUE; // to disable column resizing
    }

This works OK; but, for some reason I want to handle this message in the parent (dialog) procedure. So, I forward this message to that parent as below:

case WM_NOTIFY:
        {
            if ((((LPNMHDR)lParam)->code == HDN_BEGINTRACKA)
                || (((LPNMHDR)lParam)->code == HDN_BEGINTRACKW)) 
            {
                BOOL b = FALSE;
                HWND hParent = GetRealParent(hwnd);
                if (hParent) b = SendMessage(hParent, msg, wParam, lParam);
                return b; // to disable column resizing return TRUE;
            }
        }
        break;

The message is sent OK but, even though I return TRUE from the dialog procedure, here in the ListView procedure, the return value of the SendMessage call is FALSE.

In the dialog procedure, the code is as below:

case WM_NOTIFY:
    {
        if ((((LPNMHDR)lParam)->code == HDN_BEGINTRACKA)
            || (((LPNMHDR)lParam)->code == HDN_BEGINTRACKW))
            return TRUE;
    }

So, my question is why directly sending (forwarding) the WM_NOTIFY message to the parent returns a different result, or simply just doesn't work?

Edit :-

In the past, I have faced the same problem; to solve it, I tried a user-defined message, like:

#define UWM_WM_NOTIFY (WM_APP   7)

and use that with SendMessage to communicate between child and parent, or between any other dialogs. But it also fails to get proper return values.

So, I am using SendMessage as follow:

BOOL b = FALSE;
SendMessageA(hDlg, UWM_ANY_WM, 0, (LPARAM) &b);
return b;

Sending address of variable as LPARAM to get return value. Is there any better way to do this. Or Why SendMessageA return value is different?

CodePudding user response：

From the Microsoft documentation for the WM_NOTIFY message¹:

If the message handler is in a dialog box procedure, you must use the SetWindowLong function with DWL_MSGRESULT to set a return value.

So, using the more up-to-date SetWindowLongPtr function, your parent (dialog box) handling for the WM_NOTIFY message should look something like this:

case WM_NOTIFY:
    {
        if ((((LPNMHDR)lParam)->code == HDN_BEGINTRACKA)
            || (((LPNMHDR)lParam)->code == HDN_BEGINTRACKW))
        {
            SetWindowLongPtr(hWnd, DWL_MSGRESULT, TRUE); // hWnd is dialog's HWND
            return TRUE;
        }
    }

Also note that your handler should continue to return TRUE, as noted in this document:

If you use SetWindowLong with the DWL_MSGRESULT index to set the return value for a message processed by a dialog procedure, you should return TRUE directly afterward. Otherwise, if you call any function that results in your dialog procedure receiving a window message, the nested window message could overwrite the return value you set using DWL_MSGRESULT.

¹ Actually, you need to use the outlined mechanism to set the return value for (almost) any message handled by a dialog procedure, as nicely described in this blog by Raymond Chen.