DOCUMENT:Q185730  18-MAY-1998  [vbwin]
TITLE   :HOWTO: Prevent Multiple Instances of a 32-bit VB Application
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:4.0,5.0
OPER/SYS:WINDOWS
KEYWORDS:

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The information in this article applies to:

 - Microsoft Visual Basic Control Creation, Learning, Professional, and
   Enterprise Editions for Windows, version 5.0
 - Microsoft Visual Basic Standard, Professional, and Enterprise Editions,
   32-bit only, for Windows, version 4.0
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SUMMARY
=======

This article describes how to avoid loading a second instance of an
application when the user already has one instance running. It also sets
the focus to the first instance of the Visual Basic application when
you attempt to start a second instance of the same application.

MORE INFORMATION
================

Step-by-Step Example
--------------------

1. Create a new Visual Basic project.

2. Paste the following code into the code Module for Form1:

      Option Explicit

      Private Sub Form_Load()
         If App.PrevInstance Then
            ActivatePrevInstance
         End If
      End Sub

3. Add a Standard Module to the Project.

4. Paste the following code into the module:

      Option Explicit

      Public Const GW_HWNDPREV = 3

      Declare Function OpenIcon Lib "user32" (ByVal hwnd As Long) As Long
      Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
         (ByVal lpClassName As String, ByVal lpWindowName As String) _
         As Long
      Declare Function GetWindow Lib "user32" _
         (ByVal hwnd As Long, ByVal wCmd As Long) As Long
      Declare Function SetForegroundWindow Lib "user32" _
        (ByVal hwnd As Long) As Long

      Sub ActivatePrevInstance()
         Dim OldTitle As String
         Dim PrevHndl As Long
         Dim result As Long

         'Save the title of the application.
         OldTitle = App.Title

         'Rename the title of this application so FindWindow
         'will not find this application instance.
         App.Title = "unwanted instance"

         'Attempt to get window handle using VB4 class name.
         PrevHndl = FindWindow("ThunderRTMain", OldTitle)

         'Check for no success.
         If PrevHndl = 0 Then
            'Attempt to get window handle using VB5 class name.
            PrevHndl = FindWindow("ThunderRT5Main", OldTitle)
         End If


         'Check if found
         If PrevHndl = 0 Then
            'No previous instance found.
            Exit Sub
         End If

         'Get handle to previous window.
         PrevHndl = GetWindow(PrevHndl, GW_HWNDPREV)

         'Restore the program.
         result = OpenIcon(PrevHndl)

         'Activate the application.
         result = SetForegroundWindow(PrevHndl)

         'End the application.
         End
      End Sub

5. Compile the project into an EXE.

6. Exit Visual Basic.

7. Run the executable you created.

8. Repeat step 7.

   RESULT: The first instance of the program is given focus and the second
   instance is closed. If the first instance of the application was
   minimized, it will be restored to a normal window automatically.

REFERENCES
==========

For information about preventing multiple instances of a Visual Basic 4.0
16-bit application, please see the following article in the Microsoft
Knowledge Base:

   ARTICLE-ID: Q142937
   TITLE     : How to Prevent Multiple Instances of a VB Application

Additional query words: kbSDKWin32 kbAPI kbdSupport kbdsd
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Version           : WINDOWS:4.0,5.0
Platform          : WINDOWS
Issue type        : kbhowto

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Copyright Microsoft Corporation 1998.