How to Display a Task Modal Dialog from a 32-Bit DLL

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 32-bit only, for Windows, version 4.0

SUMMARY

If a Message box is displayed from a 32-bit DLL by calling it from Visual Basic version 4.0, the Message box does not behave as if it were a task modal dialog. In other words, Visual Basic allows you to change the focus to other forms in the application by clicking them.

On the other hand, a MsgBox displayed from Visual Basic itself always behaves as if it were a task modal dialog. This article shows by example how to force a Message box invoked from a 32-bit DLL to behave as a Visual Basic MsgBox behaves.

MORE INFORMATION

All Visual Basic applications have a background window that handles all messages for the application and all calls to the VB40032.DLL. This background window is also the owner and parent of all non-MDI child forms. You can use the GetWindow API function to get the handle of this background window, and subsequently pass this handle to the Message box called from the 32-bit DLL, so that it is also owned by the Visual Basic background window.

NOTE: You can also make the Message box task-modal in the DLL itself by calling it with a hWndOwner (first parameter) of NULL and a fuStyle (last parameter) of MB_TASKMODAL.

Step-by-Step Example

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add another form (Form2) by choosing Form from the Insert menu.

3. Add the following code to the General Declarations section of Form1:

      Private Declare Function GetWindow Lib "user32" (ByVal hWnd As _
         Long, ByVal wCmd As Long) As Long
   
      Private Declare Sub ShowTaskModal Lib "appmodal" (ByVal hWnd As Long)
   
      Private hWndVBOwner As Long
      Private Const GW_OWNER As Long = 4
   
   

4. Add the following code to the Form1_Load event:

   Form2.Show hWndVBOwner = GetWindow(Me.hWnd, GW_OWNER)

5. Add the following code to the Form_Click event:

   ShowTaskModal hWndVBOwner

6. Using Microsoft Visual C++ version 2.x:

   a. Compile a 32-bit DLL that contains the following function:

         #include <windows.h>
   

         //prototype of exported function
         void _stdcall ShowTaskModal (HWND hWnd);
   

         void _stdcall ShowTaskModal (HWND hWnd)
         {
            MessageBox (hWnd, "Hi! I am Task Modal!",
               "Task Modal Dialog From 32-bit DLL", MB_OK);
         }
   
      b. Use a definition file (.DEF) for the DLL as follows:
   
         LIBRARY AppModal
   
         CODE PRELOAD MOVEABLE DISCARDABLE
         DATA PRELOAD SINGLE
   
         EXPORTS
         ShowTaskModal @1
   
      c. Build the DLL. Call it APPMODAL.DLL, and move it to the system
         directory.
   
   

7. Go back to Visual Basic, and press the F5 key to run the program. Click Form1. The Message box from the DLL display. Try to click Form2 or any other window belonging to the Visual Basic design environment itself. You will hear a beep prohibiting you from doing so until you click the OK button of the Message box.