App.PrevInstance not refreshing itself - vb6

Question

I am trying to check to see whether another instance of the application is already running. If it does, I want to keep on checking for another 15 seconds or so before going on...
if App.PrevInstance then
dim dtStart as date
dtStart = now
do while datediff("s", dtStart, Now) < 15
Sleep 1000 ' sleep for a second
if not App.PrevInstance then exit do
loop
end if
The problem is App.PrevInstance does not seem to refresh itself. it keeps the initial value no matter what.
Is there another way to approach this? Perhaps with API calls. Note that the application may or may not have a window, thus I can't check for an existence of a window with a certain caption.

Answer 1

You might want to give this a look: http://www.codeguru.com/forum/showthread.php?t=293730

Answer 2

I use the following class:
'--------------------------------------------------------------------------------------- ' Module : CApplicationSingleton ' DateTime : 24/03/2006 15:16 ' Author : Fernando ' Purpose : Enforces a single instance of an application. Uses a Mutex ' see http://www.codeguru.com/forum/showthread.php?s=&threadid=293730 ' http://www.codeguru.com/Cpp/W-P/system/processesmodules/article.php/c5745/ ' Copyright © 2001-2007 AGBO Business Architecture S.L. '---------------------------------------------------------------------------------------
Option Explicit
Private Declare Function CreateMutex Lib "kernel32" Alias "CreateMutexA" (lpMutexAttributes As SECURITY_ATTRIBUTES, ByVal bInitialOwner As Long, ByVal lpName As String) As Long Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Private Type SECURITY_ATTRIBUTES
nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type
Private Const ERROR_ALREADY_EXISTS = 183&
Private m_hMutex As Long Private m_lLastDllError As Long
Private Sub Class_Initialize() '
On Error GoTo errorBlock
'
Dim s As SECURITY_ATTRIBUTES
m_hMutex = CreateMutex(s, 0, rcString(8700)) m_lLastDllError = Err.LastDllError
'
exitBlock:
Exit Sub
errorBlock:
Call GError.handleError(Err.Number, Err.Description, Erl, "CApplicationSingleton", "Class_Initialize", GApp.copyDebugFiles())
Resume exitBlock
' End Sub
Private Sub Class_Terminate() On Error GoTo errorBlock
If m_hMutex > 0 Then
Call CloseHandle(m_hMutex) End If
exitBlock:
Exit Sub
errorBlock:
Call GError.handleError(Err.Number, Err.Description, Erl, "CApplicationSingleton.cls", "Class_Terminate")
Resume exitBlock
End Sub
Public Function IsAnotherInstanceRunning() As Boolean '
On Error GoTo errorBlock
'
IsAnotherInstanceRunning = (m_lLastDllError = ERROR_ALREADY_EXISTS)
'
exitBlock:
Exit Function
errorBlock:
Call GError.handleError(Err.Number, Err.Description, Erl, "CApplicationSingleton", "IsAnotherInstanceRunning", GApp.copyDebugFiles())
Resume exitBlock
' End Function

Answer 3

I used the Mutex class to work out the same issue with starting the same app multiple times. It appeared to be working then stopped working returning a false positive. What I found is that the vb6 IDE was also holding a mutex while the IDE was still open.
You've gotta use the code and compile it. The EXE will work fine after you close the IDE.. Who knew? Drove me crazy(ier) for a few minutes..
I'll post a sample if anyone wants it.