Detecting a previous instance of my program

SBSteve · May 10th, 2009, 08:05 PM

How can I prevent a program from opening a second instance of itself? I would like to maximize the running instance if it exists.

alliancejhall · May 10th, 2009, 08:52 PM

Just check the processes running and then compare them to the one trying to start up. If it's already listed then kill the one trying to run.

Code:

        Dim appProc() As Process
        Dim strModName As String
        Dim strProcName As String

        strModName = Process.GetCurrentProcess.MainModule.ModuleName
        strProcName = System.IO.Path.GetFileNameWithoutExtension(strModName)

        appProc = Process.GetProcessesByName(strProcName)
         If appProc.Length > 1 Then
            MsgBox("This program is already running. You can't open more than one instance.", MsgBoxStyle.OkOnly, "Error")
            Application.Exit()
        End If

philip_cole · May 11th, 2009, 05:01 AM

Hi Steve,

I don't know if it's a preferred or better way to Jason's but my C# Programmers Cookbook suggests using a Mutex (http://msdn.microsoft.com/en-us/libr...ing.mutex.aspx) to control if an application can run. This can be useful in other scenarios as multiple programs can use it.

vbnet Code:

Imports System.Threading

Module Module1

    Sub Main()
        Dim ownsMutex As Boolean

        ' try to create a named system mutex
        Using mutex As New Mutex(True, "MutexExample", ownsMutex)
            If ownsMutex Then
                ' got it - run program
                Console.WriteLine("I own the mutex. Press Enter to release")
                Console.ReadLine()
                ' very important to release it once done!!!
                mutex.ReleaseMutex()
            Else
                ' program already running. quit
                Console.WriteLine("App is already running. Exiting...")
            End If

            Console.WriteLine("Press Enter to quit")
            Console.ReadLine()
        End Using
    End Sub

End Module

Phil

SBSteve · May 11th, 2009, 05:32 PM

Thanks for the answers. alliancejhall's example worked for me as far as detecting the running instance, but I am still having a problem with getting the running instance to restore its window if minimized or hidden.

alliancejhall · May 11th, 2009, 08:16 PM

This is totally untested but it might get you started..

Code:

    Declare Function ShowWindow Lib "user32" (ByVal winHandle As IntPtr, ByVal nCmdShow As Long) As Long

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        CheckAppIsRunning()
    End Sub

    Private Sub CheckAppIsRunning(Optional ByVal Arg As String = Nothing)

        Dim appProc() As Process

        Dim strModName As String
        Dim strProcName As String
        strModName = Process.GetCurrentProcess.MainModule.ModuleName
        strProcName = System.IO.Path.GetFileNameWithoutExtension(strModName)

        appProc = Process.GetProcessesByName(strProcName)

        Dim mProcess As Process = Nothing
        For Each p As Process In appProc
            If mProcess Is Nothing Then
                mProcess = p
            Else
                If p.StartTime < mProcess.StartTime Then mProcess = p
            End If
        Next

        Dim winHandle As IntPtr = mProcess.MainWindowHandle

        If appProc.Length > 1 Then
            Call ShowWindow(winHandle, 1)
            MsgBox("This program is already running. You can't open more than one instance.", MsgBoxStyle.OkOnly, "Program Error")
            Application.Exit()
        End If

    End Sub

SBSteve · May 12th, 2009, 01:09 AM

Thanks for your help Jason. I tried your code, but the window only appears for a second and then disappears behind other windows. I tried several values for the showWindow() function's second parameter, but never got a window that would stay on top.
By the way, is there a list of the values for the second parameter?

Steve Lyons

alliancejhall · May 12th, 2009, 08:20 AM

I should have included that.

MAXIMIZE = 3
RESTORE = 9
NORMAL = 1
SHOW = 5

These are all the constants you would need i would suggest doing something like

Call ShowWindow(winHandle, 1)
Call ShowWindow(winHandle, 5)

or

Call ShowWindow(winHandle, 9)
Call ShowWindow(winHandle, 5)

By combining the calls you are bringing up the window then showing it which activates it.