Using DWL_USER to Access Extra Bytes in a Dialog Box

• Microsoft Windows Software Development Kit (SDK) versions 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

Windows extra bytes are used to store private information specific to an instance of a window. For dialog boxes, these extra bytes are already allocated by the dialog manager. The offset to the extra byte location is called DWL_USER.

DWL_USER is the 8th byte offset of the dialog extra bytes. The programmer has 4 bytes (a long) available from this offset for personal use.

CAUTION: DO NOT use more than 4 bytes of these extra bytes, as the rest of them are used by the dialog manager.

Example

   DWORD dwNumber = 10;
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   case WM_INITDIALOG:
           SetWindowLong(hWnd,DWL_USER,dwNumber); // Store value 10 at
                                                  // byte offset 8
           dwNumber = GetWindowLong(hWnd,DWL_USER); // Retrieve the value