VB.NET: Single Instance Application

[Shuja Ali]

In VB 6, we had a very friendly App object which would give us lot of information regarding the current application. If one wanted to check if there is already an instance of this application running it was just two lines of code. In VB.NET 2002/2003 we do not have any such object so to check whether we already have an instance of the application running we need to do some extra work.

Here is a function which checks if the instance of the application is already running. If returns a Process object which contains information about the currently running instance or a null value(if the application is not running).

'Function : checkInstance
    'Input Parameter : None
    'Return Value : Returns the Process if the exe 
    'is already running or else returns nothing
    Public Function checkInstance() As Process
        Dim cProcess As Process = process.GetCurrentProcess()
        Dim aProcesses() As Process = process.GetProcessesByName(cProcess.ProcessName)
        'loop through all the processes that are currently running on the 
        'system that have the same name
        For Each process As Process In aProcesses
            'Ignore the currently running process 
            If process.Id <> cProcess.Id Then
                'Check if the process is running using the same EXE as this one
                If [Assembly].GetExecutingAssembly().Location = cProcess.MainModule.FileName Then
                    'if so return to the calling function with the instance of the process
                    Return process
                End If
            End If
        Next
        'if nothing was found then this is the only instance, so return null
        Return Nothing
    End Function

We just have to call this function at the entry point of our application. If the return value is Null then this is the first instance and if it returns a process then we already have an instance of the application running on the system. Ideally a project will be started from Sub Main, so I would put the code in the Sub Main procedure.

Public Sub main()
        Dim tempProcess As Process
        tempProcess = checkInstance()
        If tempProcess Is Nothing Then
            Application.Run(New Form1)
        Else
            MessageBox.Show("Application is already running")
        End If
    End Sub

In case you want to set focus to the currently running instance then you can get the MainWindowHandle of the already running instance like this

tempProcess.MainWindowHandle

And then use SetForeGroundWindow Api to activate the already running instance.

Don't forget to Import System.Diagnostics namespace.