Microsoft KB Archive/185730

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

Article ID: 185730

Article Last Modified on 5/3/2005

-

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

-



This article was previously published under Q185730



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.



Step-by-Step Example
 Create a new Visual Basic project. Go into Project Properties and set the Startup Object (or Startup Form in VB4) to be Sub Main.

 Add a Standard Module to the Project.  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

 Compile the project into an EXE. Exit Visual Basic. Run the executable you created.</li> 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.</li></ol>

<div class="references_section">