HOWTO: Drawing a Different Icon for Each Application Instance

• Microsoft Windows Software Development Kit (SDK) 3.1

SUMMARY

An application can use the DrawIcon function in its paint procedure to display a different icon for each application instance. The method has the following five steps.

MORE INFORMATION

1. In the WNDCLASS data structure used to register the application's main window class, set the hIcon field to NULL. This prevents Windows from drawing an icon when the application is minimized. The following code demonstrates this step:

   
      int index;  // global variable to count application instances
   
      // This function initializes the first application instance
      void InitFirst(HANDLE hInstance)
      {
         WNDCLASS rClass;            // window class structure
   
         rClass.lpszClassName = "HELLO";
         rClass.hInstance     = hInstance;
         rClass.lpfnWndProc   = WindowProc;
         rClass.hCursor       = LoadCursor(NULL, IDC_ARROW);
   
         // If hIcon is NULL, Windows will not draw icon. Application
         // paints its own icon as appropriate.
         rClass.hIcon         = NULL;
   
         rClass.lpszMenuName  = NULL;
         rClass.hbrBackground = COLOR_WINDOW + 1;
         rClass.style         = CS_HREDRAW | CS_VREDRAW;
         rClass.cbClsExtra    = 0;
         rClass.cbWndExtra    = 0;
   
         RegisterClass(&rClass);     //  register the class
   
         index = 1;  // set counter to indicate first instance
      } 




2. The GetInstanceData function provides the value of the instance counter from the previous instance. Adding one to this value provides the value for the current instance, as follows:

   
      void InitNext(HANDLE hInstance, HANDLE hPrevInstance)
      {
         GetInstanceData(hPrevInstance, (PSTR)&index, sizeof(int));
         index++;
      } 




3. During the instance initialization function, load the icon for the current instance from the application's resources. In this case, Icon1, Icon2, Icon3, and Icon4 are individual icons declared in the application's resources. The following code demonstrates this procedure:

   
      HICON hIcon; // icon handle for this instance
   
      void InitEvery(HANDLE hInstance, int cmdShow)
      {
         HWND  hWnd;
   
         hInst = hInstance; // store instance in global variable for
                            // window procedures to use
   
         switch (index)
            {
         case 1: hIcon = LoadIcon(hInstance, "Icon1");
                 break;
   
         case 2: hIcon = LoadIcon(hInstance, "Icon2");
                 break;
   
         case 3: hIcon = LoadIcon(hInstance, "Icon3");
                 break;
   
         default:
                 hIcon = LoadIcon(hInstance, "Icon4");
            }
   
         CreateWindow( ... ); // and so forth
      } 




4. When an application receives a WM_PAINT message while the window is minimized, draw the icon in the application's client area. The IsIconic function returns TRUE if the application is minimized. The following code demonstrates this step:

   
      // In the switch(message) statement of the main window procedure
   
      case WM_PAINT:
         {
         PAINTSTRUCT ps;
   
         if (IsIconic(hWnd))
            {
            BeginPaint(hWnd, (LPPAINTSTRUCT)&ps);
   
            // Paint the desktop window background.
            DefWindowProc(hWnd, WM_ICONERASEBKGND, (WORD)ps.hdc, 0L);
   
            // Draw the icon in the cleared area
            DrawIcon(ps.hdc, 0, 0, hIcon);
   
            EndPaint(hWnd, (LPPAINTSTRUCT)&ps);
            }
         else
            // Paint the window as usual
         } 




5. When the user drags an application's icon with the mouse and the application processes the WM_QUERYDRAGICON message as follows, Windows changes the mouse cursor to a black-and-white representation of the application's icon.

   
      // In the switch(message) statement of the main window procedure
   
      case WM_QUERYDRAGICON:
         return (LONG)(WORD)hIcon;
         break; 

Additional query words: 3.00 3.10

Keywords : kb16bitonly kbSDKWin16
Version : WINDOWS:3.1
Platform : WINDOWS
Issue type : kbhowto