Classic VB - How can I allow only one instance of my application to run at a time?

[dee-u]

Method 1: You can read the PrevInstance property of the App object, if the value of this is True then another instance of the application is already running.

If your program starts with Sub Main, you can use this code to exit the program if another copy is already running:'in sub main...
If App.PrevInstance = True Then
MsgBox "Already running...."
Exit Sub
End If
For forms you need an extra piece of code to also close the form, the following code should be placed in Form_load:'in form_load...
If App.PrevInstance = True Then
MsgBox "Already running...."
Unload Me
Exit Sub
End If


Method 2: You can use the FindWindow API to search for open windows with your form caption.

Note that for this to work you must know the exact title to find, so if you change your window title within your program

(such as adding a file name, like Notepad does) then this will fail. Also, if other running programs have the same caption,

your program may not run at all.

Option Explicit

Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As

String) As Long

Public Sub Main()
Dim m_hWnd As Long

'Change Form1 to your form Caption.
m_hWnd = FindWindow(vbNullString, "Form1")
If m_hWnd > 0 Then
MsgBox "Program " & App.Title & " is already running..."
Exit Sub
End If

'Change Form1 to your form name.
Form1.Show
End Sub

[Habin]

'Module
Public Declare Function ReleaseMutex Lib "kernel32" (ByVal hMutex As Long) As Long
Public Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
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 FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function SetForegroundWindow Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
Private Declare Function FlashWindowEx Lib "user32.dll" (ByRef pfwi As FLASHWINFO) As Long


Private Const FLASHW_STOP = 0
Private Const FLASHW_CAPTION = &H1
Private Const FLASHW_TRAY = &H2
Private Const FLASHW_ALL = (FLASHW_CAPTION Or FLASHW_TRAY)
Private Const FLASHW_TIMER = &H4

Private Const ERROR_ALREADY_EXISTS = 183&
Private Const SW_RESTORE = 9
Private Type SECURITY_ATTRIBUTES
nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type

Private Type FLASHWINFO
cbSize As Long
hwnd As Long
dwFlags As Long
uCount As Long
dwTimeout As Long
End Type


Public Mutex As Long

Private Sub Main()
Dim sa As SECURITY_ATTRIBUTES
Dim hwnd As Long
Dim FlashInfo As FLASHWINFO

sa.bInheritHandle = 1
sa.lpSecurityDescriptor = 0
sa.nLength = Len(sa)
Mutex = CreateMutex(sa, 1, App.Title)
If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
hwnd = GetSetting("AppName", "Section", "Key", 0)
ShowWindow hwnd, SW_RESTORE
SetForegroundWindow hwnd

FlashInfo.cbSize = Len(FlashInfo)
FlashInfo.dwFlags = FLASHW_ALL Or FLASHW_TIMER
FlashInfo.dwTimeout = 0
FlashInfo.hwnd = hwnd
FlashInfo.uCount = 3
FlashWindowEx FlashInfo
Else
FrmMain.Show
End If
End Sub





'Form
Private Sub Form_Load()
SaveSetting "AppName", "Section", "Key", Me.hwnd
End Sub

Private Sub Form_Unload(Cancel As Integer)
ReleaseMutex Mutex
CloseHandle Mutex
DeleteSetting "AppName", "Section", "Key"
End Sub