How to Restrict the Application to Just One Instance in C#

You can achieve it using threading.

More info on threading visit Microsoft.com

using System.Threading;

[STAThread]

static void Main()

{

    using (Mutex mutex = new Mutex(false, "TestApplication"))

    {

        if (!mutex.WaitOne(0, false))

        {

            MessageBox.Show("Instance already running");

            return;

        }

        Application.EnableVisualStyles();

        Application.SetCompatibleTextRenderingDefault(false);

        Application.Run(new Form1());

        mutex.ReleaseMutex();

    }

}

OR

[STAThread]

static void Main()

{

    bool createdNew;

    using (Mutex mutex = new Mutex(false, "TestApplication", out createdNew))

    {

        if (createdNew)

        {

            MessageBox.Show("Instance already running");

            return;

        }

        Application.EnableVisualStyles();

        Application.SetCompatibleTextRenderingDefault(false);

        Application.Run(new Form1());

    }

}