MainWndProc (HWND hwnd, UINT nMsg, WPARAM wParam, LPARAM
lParam)
{
        /* The window handle for the "Click Me" button. */
static HWND hwndButton = 0;
static int   cx, cy; /* Height and width of our button. */
     HDC             hdc;   /* A device context used for drawing */
     PAINTSTRUCT     ps;     /* Also used during window drawing */
    RECT            rc;  /* A rectangle used during drawing */

        /*
         * Perform processing based on what kind of message we got.
         */
        switch (nMsg)
        {
                case WM_CREATE:
                 {
                        /* The window is being created. Create our button
                         * window now. */
                        TEXTMETRIC   tm;

                        /* First we use the system fixed font size to choose
                         * a nice button size. */
                         hdc = GetDC (hwnd);
     SelectObject (hdc, GetStockObject (SYSTEM_FIXED_FONT));
                        GetTextMetrics (hdc, &tm);
                        cx = tm.tmAveCharWidth * 30;
                        cy = (tm.tmHeight + tm.tmExternalLeading) * 2;
                        ReleaseDC (hwnd, hdc);

                        /* Now create the button */
                        hwndButton = CreateWindow (
                                "button",    /* Builtin button class */
                                "Click Here",
                                WS_CHILD | WS_VISIBLE | BS_PUSHBUTTON,
                                0, 0, cx, cy,
                                hwnd,        /* Parent is this window. */
                                (HMENU) 1,   /* Control ID: 1 */
                                ((LPCREATESTRUCT) lParam)->hInstance,
                                NULL
                                );

                        return 0;
                    break;
                }

                case WM_DESTROY:
                    /* The window is being destroyed, close the application
                     * (the child button gets destroyed automatically). */
                        PostQuitMessage (0);
                        return 0;
                        break;

                case WM_COMMAND:
                        /* Check the control ID, notification code and
                         * control handle to see if this is a button click
                         * message from our child button. */
                        if (LOWORD(wParam) == 1 &&
                            HIWORD(wParam) == BN_CLICKED &&
                            (HWND) lParam == hwndButton)
                        {
                                /* Our button was clicked. Close the window. */
                                 DestroyWindow (hwnd);
                        }
                        return 0;
                        break;
        }

        /* If we don't handle a message completely we hand it to the system
         * provided default window function. */
        return DefWindowProc (hwnd, nMsg, wParam, lParam);
}