Security Blackout Window

Introduction

Someone posted a question in Q&A recently, asking how to create the semi-transparent "blackout" window on the screen in the same way that Windows does when it asks you whether to allow a program to proceed or not. I responded by saying that all you need is another window on top of the original, and a dialog box with the question. Having thought a bit more about the problem I wondered whether that would actually work or not. After some investigation I found the answer was actually fairly simple, once I had figured out how to create and manage the secondary window.

Using the code

The code below is fairly self-explanatory but a few notes may help:

• First we need to register a new Windows class with the following attributes:
• No special class styles, this window does nothing much.
• Use DefWindowProc as its message handler, as our code will not deal with any messages.
• Set the backgound colour to the system colour value COLOR_DESKTOP, which is black on my system.
• Get the dimensions of the caller's window (see note below) and create a Window of our new class with the following attributes:
• Extended style of WS_EX_LAYERED, this is a layered window with no frame borders.
• Styles of WS_VISIBLE | WS_POPUP, this is a visible popup.
• The same size and position as the caller's window.
• Its parent must be the caller's Window, to make the parent inaccessible.
• Next we call SetLayeredWindowAttributes to make the overlay completely obscure the parent window.
• Sleep for half a second. Yes, this is fluff.
• Call SetLayeredWindowAttributes again to make the overlay semi-transparent.
• Show a dialog: I have used the system TaskDialog to display the user's message text.
• When complete, destroy the window, unregister the class and return the response to the caller.

In my code, if the caller does not provide a Window handle then we get the handle of the Desktop Window and completely cover the screen.

#include <Windows.h>
#include <tchar.h>
#include <Commctrl.h>

/// <summary>
/// This function registers and creates a simple overlay Window
/// that is used to obscure the current window or the entire Desktop
/// until a dialog has been responded to.
/// </summary>
///
/// <param name="hParentWnd">Handle to the parent window to be obscured</param>
/// <param name="szTitle">optional title of the dialog box</param>
/// <param name="szMain">Main line of text</param>
/// <param name="szContent">supplementary dialog text</param>
///
/// <returns>TRUE or FALSE, depending on the user's response</returns>
///
BOOL StopDialog(HWND  hParentWnd,
                PTSTR  szTitle,
                PTSTR  szMain,
                PTSTR  szContent
                )
{
    WNDCLASSEX      wndClass;        // class structure to register the obscuring window
    HWND            hStopWnd;        // handle to the obscuring window
    RECT            rectParent;      // to get the dimensions of the parent window
    int             nButton = IDNO;  // default answer
    // Register the window class
    wndClass.cbSize        = sizeof wndClass;                   //
    wndClass.style         = 0;                                 // no  styles for this class
    wndClass.lpfnWndProc   = DefWindowProc;                     // we will not handle any messages
    wndClass.cbClsExtra    = 0;                                 // no extra space required
    wndClass.cbWndExtra    = 0;                                 //
    wndClass.hInstance     = GetModuleHandle(NULL);             // instance handle to the current process
    wndClass.hIcon         = NULL;                              // no icon
    wndClass.hCursor       = NULL;                              //    or cursor
    wndClass.hbrBackground = GetSysColorBrush(COLOR_DESKTOP);   // a black brush
    wndClass.lpszMenuName  = NULL;                              // no menu
    wndClass.lpszClassName = _T("Blackout");                    // our class name
    wndClass.hIconSm       = NULL;                              // no small icon
    if (RegisterClassEx(&wndClass))
    {
        if (hParentWnd == NULL)
        hParentWnd = GetDesktopWindow();                   // if no window given, then we will use the entire desktop
        GetWindowRect(hParentWnd, &rectParent);            // get the dimensions of the parent window
        int nWidth = rectParent.right - rectParent.left;
        int nHeight = rectParent.bottom - rectParent.top;
        // create the overlay window
        hStopWnd = CreateWindowEx(         // create a new window same size and position as the parent
            WS_EX_LAYERED,                 // this is a layered window
            wndClass.lpszClassName,        // registered class name
            NULL,                          // no title necessary
            WS_VISIBLE | WS_POPUP,         // it's a popup and always visible
            rectParent.left,               // horizontal position of window
            rectParent.top,                // vertical position of window
            nWidth,                        // window width
            nHeight,                       // window height
            hParentWnd,                    // handle to parent or owner window
            NULL,                          // no menu handle
            wndClass.hInstance,            // handle to application instance
            NULL                           // no create parameters
            );
        if (hStopWnd)
        {
            // we have our overlay window so set it to totally obscure its parent
            SetLayeredWindowAttributes(hStopWnd, 0, 255, ULW_ALPHA);
            // wait half a second
            Sleep(500);
            // now set it less opaque and show the response dialog
            SetLayeredWindowAttributes(hStopWnd, 0, 196, ULW_ALPHA);
            // We use a standard Windows TaskDialog control as it is far simpler
            // than creating a custom dialog box, although that is still an option.
            HRESULT hResult = TaskDialog(hStopWnd,
                NULL,                                // no hInstance as we use system icons and buttons
                szTitle,                             // title provided by the caller
                szMain,                              // main message ditto
                szContent,                           // secondary message ditto
                TDCBF_YES_BUTTON | TDCBF_NO_BUTTON,  // use the Yes and No buttons
                TD_SHIELD_ICON,                      // and system shield icon
                &nButton                             // response stored in here
                );
            if (hResult != S_OK)  // if the dialog fails for any reason, we answer No
                nButton = IDNO;
            DestroyWindow(hStopWnd); // finished with our obscuring window
        }
        UnregisterClass(wndClass.lpszClassName, wndClass.hInstance); // and finished with our class
    }
    return nButton == IDYES ? TRUE : FALSE;  // return TRUE if the user answers Yes, otherwis FALSE
} 

History

• Initial post 27 October 2013
• Cosmetic changes: 20 November 2013
• Added sample image: 20 November 2013.