Easily Create A Single Instance Application

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Introduction

Sometimes you want your application to run single instance. For example, if you wrote a server application you probably only want a single instance to run at a time on the localhost.

This code was developed primarily for a server application running on a Win2KPro Multi-Processor server. It is intended to make absolutely certain that only one instance of the application runs at a time regardless of logged on users. The code detects all processes running in all desktops and all service executables as well and will not start another process if one is already running.

There is an easy way to start an application so that it first checks to see if another application (process) of the same name is already running.

Create a new module as below and:

1) Add the module to your WinForm application project
2) Change the 'MyAppName' occurences as appropriate
3) Change the name of the form to start the application with [Application.Run(New MainForm)]
4) Change the StartupObject in the project properties to Main()

That's all it takes. Have fun.

Take a look at the code:

<PRE lang=vb.net>Imports System.Diagnostics 
Module SingleInstance

   'Entry point for the application. Main checks for a prior instance and closes
   'this one if it finds a prior one running.
   Public Sub Main() 

        If CheckForDuplicateProcess("MyAppName") Then
            Dim dupProcess As String
            dupProcess = "There is another instance of MyAppName" & _
        " running on this machine." & vbCrLf & _
        "This new instance must close." & vbCrLf & vbCrLf & _
        "Only 1 instance of MyAppName can exist" & _
        " on a machine." & vbCrLf & vbCrLf
            
        MsgBox(dupProcess, MsgBoxStyle.Critical, "Duplicate Process Detected") 

            Application.Exit() 

        Else 
                 '****** Change The FormName Below *****
                 'Change the name of the form to load, to the one 
                 'applicable for your application
            Application.Run(New MainForm)
    End If 

End Sub

'Determines if there already is a 'processName' running in the local host.
'Returns true if it finds more than 'one processName' running
Private Function CheckForDuplicateProcess(ByVal processName As String) As Boolean
     'function returns true if it finds more than one 'processName' running
     Dim Procs() As Process
     Dim proc As Process

'get ALL processes running on this machine in all desktops
'this also finds all services running as well.

        Procs = Process.GetProcesses()
        Dim count As Integer = 0
        For Each proc In Procs
            If proc.ProcessName.ToString.Equals(processName) Then
               count += 1
            End If
        Next proc
       If count > 1 Then
            Return True
        Else
           Return False
        End If
    End Function
End Module

The example code has been tested on Win2K Pro and WinXP Pro. In all cases
it works as expected. No failures of any kind thus far.

Some readers have had a few comments and I have tried to reply to them below.

ProcessName Truncation By OS

The Process.ProcessName that CheckForDuplicateProcess() uses to loop thru all running processes is the same as the process name used in Task Manager. So if you have an exe named me.exe, and if you start two instances of it, you'll see two processes called 'me' in the task manager. They are not truncated in the task manager so using ProcessName works as expected.

On Using A Mutex To Obtain A Lock

I did not use the Mutex method because it would not be reliable on a multi-processor machine. If the original process was running on processor 1 and a duplicate process was started and attached to processor 2 then both would get a successful lock and you would have two processes running when you only want 1. Looping thru all the running processes is the only reliable way to prevent duplicate processes from starting. The Mutex method has less typing but is not a good solution for this problem.

On The Question Of Two Instances Starting At The Same Moment

Two instances starting at exactly the same moment on a single processor machine is impossible. And very very unlikely on a multi-processor machine. So unlikely in fact that we need not even worry about it in this situation.