|The Mutex may seem like a weird and wonderful term, but it is simply a means of creating a unique process ID within the windows operating system. You can use these values for many different means, but here it is being used to uniquely identify our application. Once the Mutex value has been created, we can then detect for its existance, and action on whether it is found.
Cut and Paste the function and variable declarations into the header of your VB application, and drop the execution code into the starting function of your application, either the form_load(), or main() method.
'DLL Function Calls
Private Declare Function CreateMutex Lib "kernel32" Alias "CreateMutexA" (ByVal lpMutexAttributes As Long, ByVal bInitialOwner As Long, ByVal lpName As String) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
'variable constant to match if the mutex exists
Private Const ERROR_ALREADY_EXISTS = 183&
'Application Variable Declarations
Public Const AppVer = "MyApp v1.1"
Dim mutexvalue As Long
'*the following code would go in the starting function of your application
'either the main() or form_load() depending on how your application works
'Create an individual mutex value for the application
mutexvalue = CreateMutex(ByVal 0&, 1, AppVer)
'If an error occured creating the mutex, that means it
'must have already existed, therefore your application
'is already running
If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
'Inform the user of running the same app twice
MsgBox "The application is already running."
'Terminate the application via the reference to it, its hObject value
This article was originally written by barnseyboy