Ensuring only one application instance [duplicate]

Answer 1

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

Answer 2

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.