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

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.

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