Ensuring only one application instance

Question

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What is the correct way to create a single instance application?

I have a Winforms app, which launches a splash screen via the following code:

Hide();
        bool done = false;
        // Below is a closure which will work with outer variables.
        ThreadPool.QueueUserWorkItem(x =>
                                  {
                                      using (var splashForm = new SplashScreen())
                                      {
                                          splashForm.Show();
                                          while (!done)
                                              Application.DoEvents();
                                          splashForm.Close();
                                      }
                                  });

        Thread.Sleep(3000);
        done = true;

The above is in the main form's codebehind and called from the load event handler.

However, how can I ensure that only one instance of the application will load at a time? In the load event handler of the main form, I could check if the process list is on the system (via GetProcessesByName(...)), but is there a better method?

Using .NET 3.5.

You should call `Application.Run(splashForm)` instead of a `DoEvents()` loop. — SLaks, Jun 26 '11 at 19:34

score 67 · Accepted Answer · answered Jun 26 '11 at 19:54

67

GetProcessesByName is slow way of checking if another instance is running. The fastest and elegant method is using mutex:

[STAThread]
    static void Main()
    {
        bool result;
        var mutex = new System.Threading.Mutex(true, "UniqueAppId", out result);

        if (!result)
        {
            MessageBox.Show("Another instance is already running.");
            return;
        }

        Application.Run(new Form1());

        GC.KeepAlive(mutex);                // mutex shouldn't be released - important line
    }

Please also bear in mind that the code you presented is not the best approach. As it was advised in one of comments calling DoEvents() in a loop is not the best idea.

answered Jun 26 '11 at 19:54

michalczerwinski

1,069
9
6

GC.KeepAlive(mutex); - doesn't work for me by some reasons. I was forsed to use private static mutex; – monstr Apr 11 '14 at 06:34
I may be wrong, but shouldn't 'GC.KeepAlive(mutex);' line be before 'Application.Run(new Form1());'? 'Application.Run()' method starts the program's message loop and does not return until 'Form1' is closed. – Alex B. Dec 20 '14 at 09:44
@AlexB.: no, this is on purpose. The goal is to prevent mutex from being released, which can occured when a new form is opened and Form1 is closed. Before Form1 is closed there is no risk of mutex being released – michalczerwinski Feb 22 '15 at 22:33
1

in if block, putting "application.exit()" instead of "return ", works form me, but i put the mutex check in the form1's form_load . – bh_earth0 Feb 24 '16 at 14:36
3

Why some people recommend use `mutex.ReleaseMutex()`, what's the difference between `GC.KeepAlive(mutex)` ? – qakmak Apr 10 '16 at 17:11

score 34 · Answer 2 · answered Jun 26 '11 at 21:19

static class Program
{
    // Mutex can be made static so that GC doesn't recycle
    // same effect with GC.KeepAlive(mutex) at the end of main
    static Mutex mutex = new Mutex(false, "some-unique-id");

    [STAThread]
    static void Main()
    {
        // if you like to wait a few seconds in case that the instance is just 
        // shutting down
        if (!mutex.WaitOne(TimeSpan.FromSeconds(2), false))
        {
            MessageBox.Show("Application already started!", "", MessageBoxButtons.OK);
            return;
        }

        try
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
        finally { mutex.ReleaseMutex(); } // I find this more explicit
    }
}

One note about the some-unique-id -> this should be unique on the machine, so use something like your company name / application name.

Edit:

http://sanity-free.org/143/csharp_dotnet_single_instance_application.html

Ensuring only one application instance

2 Answers2

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