In contrast to window procedures, which receive WM_CREATE messages, dialog procedures receive WM_INITDLG messages, which are sent after a dialog window is created, but before it is displayed. WM_INITDLG can do the same type of initialization tasks that WM_CREATE handles, but is not the first message that is received.

For example, if a dialog window contains a list box, use WM_INITDLG to fill the list box with items. Also use this procedure to enable or disable buttons in a dialog window, depending on your application.

You also can call the WinSetDlgItemText or WinSetDlgItemShort functions during dialog initialization, to set up text items that reflect the current conditions in your application.

Another typical task for the WM_INITDLG message handler is centering a dialog window on the screen or within its owner window. The following code fragment illustrates how to center a dialog window on the screen using WM_INITDLG:

    RECTL rclScreen,rclDialog;
    LONG  sWidth,sHeight,sBLCx,sBLCy;

    case WM_INITDLG:
        /* Center the dialog window and get the screen rectangle.   */
        WinQueryWindowRect(HWND_DESKTOP, &rclScreen);

        /* Get the dialog-window rectangle.                         */
        WinQueryWindowRect(hwnd, &rclDialog);

        /* Get the dialog-window width.                             */
        sWidth = (LONG) (rclDialog.xRight - rclDialog.xLeft);

        /* Get the dialog-window height.                            */
        sHeight = (LONG) (rclDialog.yTop - rclDialog.yBottom);

        /* Set the horizontal coordinate of the lower-left corner.  */
        sBLCx = ((LONG) rclScreen.xRight - sWidth) / 2;

        /* Set vertical coordinate of the lower-left corner.        */
        sBLCy = ((LONG) rclScreen.yTop - sHeight) / 2;

        /* Move, size, and show the window.                         */
        WinSetWindowPos(hwnd,
            HWND_TOP,
            sBLCx, sBLCy,
            0, 0,          /* Ignores size arguments                */
            SWP_MOVE);

        return 0;

The dialog procedure receives notification messages from each control-window item in a dialog window whenever a user clicks an item or enters text in an entry field. Most dialog procedures wait for the user to select one or more dialog-window buttons to signal being finished with the dialog window. When the dialog procedure receives one of these messages, it calls the WinDismissDlg function, as shown in the following code fragment. The second argument to WinDismissDlg is the value returned by the WinDlgBox or WinProcessDlg functions. Generally, these functions return the identifier of the button that was pressed.

    MRESULT EXPENTRY SampDialogProc(HWND hwnd,
                                    ULONG ulMessage,
                                    MPARAM mp1,
                                    MPARAM mp2)
    {
        switch (ulMessage) {
            case WM_COMMAND:
                switch (SHORT1FROMMP(mp1)) {
                    case DID_OK:

                       /*
                        * Final dialog-item queries,
                        * dismiss the dialog.
                        */

                       WinDismissDlg(hwnd, DID_OK);
                       return 0;
                }
                break;
        }
        return (WinDefDlgProc(hwnd, ulMessage, mp1, mp2));
    }

Other dialog-window items send notification messages specific to the type of control window. Your dialog procedure should respond to notification messages from any relevant or important dialog items, and pass the messages that your dialog procedure does not handle to the WinDefDlgProc function for default processing. The default dialog procedure is used similarly to the default frame-window procedure.

The WM_COMMAND message from the OK button indicates that the user has selected the OK button and is finished with the dialog window. If the dialog window has other controls, such as entry fields or check boxes, have your dialog procedure query the contents or state of each control upon receipt of a message from the OK button. Before dismissing a dialog window, have your dialog procedure collect input from each dialog-window control before closing the dialog window.