How to prevent multiple instances of a 32-bit VB application

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Article ID: 185730 - View products that this article applies to.
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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. Go into Project Properties and set the Startup Object (or Startup Form in VB4) to be Sub Main.
  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
    
          Private Sub Form_Load()
                If App.PrevInstance Then
                   ActivatePrevInstance
                End If
             End Sub
    
          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
             'Attempt to get window handle using VB6 class name
             PrevHndl = FindWindow("ThunderRT6Main", 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.
             Unload Me
          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 more information about preventing multiple instances of a Visual Basic 4.0 16-bit application, click the following article number to view the article in the Microsoft Knowledge Base:
142937 How to prevent multiple instances of a VB application

Properties

Article ID: 185730 - Last Review: May 3, 2005 - Revision: 4.0
APPLIES TO
  • Microsoft Visual Basic 6.0 Learning Edition
  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual Basic 6.0 Enterprise Edition
  • Microsoft Visual Basic 5.0 Control Creation Edition
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
  • Microsoft Visual Basic 4.0 Standard Edition
  • Microsoft Visual Basic 4.0 Professional Edition
  • Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
Keywords: 
kbhowto KB185730

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