Prevent multiple instances of a given app in .NET?

Use Mutex. One of the examples above using GetProcessByName has many caveats. Here is a good article on the subject:

http://odetocode.com/Blogs/scott/archive/2004/08/20/401.aspx

[STAThread]
static void Main() 
{
   using(Mutex mutex = new Mutex(false, "Global\\" + appGuid))
   {
      if(!mutex.WaitOne(0, false))
      {
         MessageBox.Show("Instance already running");
         return;
      }

      Application.Run(new Form1());
   }
}

private static string appGuid = "c0a76b5a-12ab-45c5-b9d9-d693faa6e7b9";

if (Process.GetProcessesByName(Process.GetCurrentProcess().ProcessName).Length > 1)
{
  AppLog.Write("Application XXXX already running. Only one instance of this application is allowed", AppLog.LogMessageType.Warn);
  return;
}

Here is the code you need to ensure that only one instance is running. This is the method of using a named mutex.

public class Program
{
    static System.Threading.Mutex singleton = new Mutex(true, "My App Name");

    static void Main(string[] args)
    {
        if (!singleton.WaitOne(TimeSpan.Zero, true))
        {
            //there is already another instance running!
            Application.Exit();
        }
    }
}