ACC2000: Determining How Many Instances of Application Are Active

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1. Start Microsoft Access 2000 and create a new, blank database called TestAPI.mdb.
2. Create a module and type the following lines in the Declarations section:

   '------------------------------------------
   ' Global Declarations Section Of The Module
   '------------------------------------------
   
   Declare Function GetWindow Lib "user32" (ByVal hwnd As Long, _
   ByVal wCmd As Long) As Long
   
   Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" _
      (ByVal hwnd As Long, ByVal lpString As String, ByVal CCh As Long) _
      As Long
   
   Declare Function IsWindowVisible Lib "user32" (ByVal hwnd As Long) _
      As Long
   
   Public Const GW_HWNDFIRST = 0
   Public Const GW_HWNDLAST  = 1
   Public Const GW_HWNDNEXT  = 2
   Public Const GW_HWNDPREV  = 3
   					

3. Type the following procedures:

   ' This function returns the Caption Text of each window passed to
   ' it. If a window does not have a Caption bar, then this function
   ' returns a zero-length string ("")
   
   Function GetAppName(Lnghwnd as long)
      Dim LngResult As Long
      Dim StrWinText As String * 255
      Dim LngCCh As Long
      LngResult = GetWindowText(Lnghwnd, StrWinText, 255)
      GetAppName = Left(StrWinText, LngResult)
   End Function
   
   ' This function counts all instances of an application that are open,
   ' including any windows that are not visible.
   ' Arguments: LngHwnd        = Any valid window handle.
   '            StrAppCaption  = The window caption to search for.
   ' Example:   GetCountOfWindows(hWndAccessApp,"Microsoft Access")
   Function GetCountOfWindows(Lnghwnd, StrAppCaption)
      Dim LngResult As Long
      Dim LngICount As Long
      Dim StrAppName As String
   
      LngResult = GetWindow(Lnghwnd, GW_HWNDFIRST)
      Do Until LngResult = 0
         If IsWindowVisible(LngResult) Then
            StrAppName = GetAppName(LngResult)
            If InStr(1, StrAppName, StrAppCaption) Then
               LngICount = LngICount + 1
            End If
         End If
         LngResult = GetWindow(LngResult, GW_HWNDNEXT)
      Loop
      GetCountOfWindows = LngICount
      End Function
   					

4. Save the module as Module1 and close it.
5. Create a new form not based on any table or query in Design view.
6. Set the OnOpen property of the form to the following event procedure:

   Private Sub Form_Open(Cancel As Integer)
   If GetCountOfWindows(hWndAccessApp, "Microsoft Access") > 1 Then
      Cancel = True
      MsgBox "Please use the instance of Microsoft Access that is " _
             & "already open."
      DoCmd.Quit acQuitSaveNone
   End If
   End Sub
   					

7. Save the form as frmStartup and close it.
8. On the Tools menu, click Startup.
9. In the Startup dialog box, select frmStartup in the Display Form box, and then click OK.
10. Close and then reopen the TestAPI database non-exclusively. Note that the frmStartup form appears.
11. Start another instance of Microsoft Access 2000 and open the TestAPI database. Note that you receive a message, and then the instance of Microsoft Access closes.