This code shows 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.
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.
End
End Sub