Microsoft KB Archive/167843

= ACC: Determining How Many Instances of Application Are Active =

Article ID: 167843

Article Last Modified on 1/19/2007

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APPLIES TO


 * Microsoft Access 95 Standard Edition
 * Microsoft Access 97 Standard Edition

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This article was previously published under Q167843



Advanced: Requires expert coding, interoperability, and multiuser skills.



SUMMARY
You can use Windows API calls in Visual Basic for Applications code to determine how many instances of an application are running. Then you can use the information to prevent re-entrance of an application that is already running.

For information about how to do this in Microsoft Access 1.x and 2.0, please see the following article in the Microsoft Knowledge Base:

96591 ACC: Determining How Many Instances of Application Are Active

This article assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to your version of the "Building Applications with Microsoft Access" manual.



MORE INFORMATION
The following example uses a procedure in the Open event of a startup form to determine if Microsoft Access is already running. If it is running, a message advises the user, and then the second instance of the program closes.  Start Microsoft Access and create a new, blank database called TestAPI.mdb.  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   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  Save the module as Module1 and close it. Create a new form not based on any table or query in Design view.  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 </li> Save the form as frmStartup and close it.</li> On the Tools menu, click Startup.</li> In the Startup dialog box, select frmStartup in the Display Form box, and then click OK.</li> Close and then reopen the TestAPI database non-exclusively. Note that the frmStartup form appears.</li> Start another instance of Microsoft Access and open the TestAPI database. Note that you receive a message, and then the instance of Microsoft Access closes.</li></ol>

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