Example usage:
   static void Main()
   {
      SingleInstanceCheck.Check();
      ...
 
 
 
Implementation:
 
/// <summary>
/// This checks for another instance of an app.
/// Use inside Main() by calling SingleInstanceCheck.Check();
/// Include this class as a sibling to Main's class.
/// If another instance of the app is detected, 
/// - A message box appears.
/// - 'this' instance calls Exit.
/// </summary>
/// <example>
///    static void Main()
///    {
///       MyApp.SingleInstanceCheck.Check();
///       Application.EnableVisualStyles();
///       Application.SetCompatibleTextRenderingDefault(false);
///       Application.Run(new Form1());
///    }
/// </example>
public static class SingleInstanceCheck
{
    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool SetForegroundWindow(IntPtr hWnd);
 
    static public void Check()
    {
        // http://snipplr.com/view/19272/ - C#, single-instance-check using mutex
        // http://iridescence.no/post/CreatingaSingleInstanceApplicationinC.aspx
        bool isOwnedHere = false;
        appStartMutex = new System.Threading.Mutex(
            true,
            Application.ProductName,
            out isOwnedHere
        );
 
        if (!isOwnedHere) // if owned by other process.
        {
            string message = string.Format("There is already a copy of the application {0} running.\n\nClick cancel to exit.", Application.ProductName);
 
            DialogResult x = MessageBox.Show(message, Application.ProductName + " already running", MessageBoxButtons.OKCancel);
 
            if (x == DialogResult.Cancel) // Cancel, but show the already-existing process.
            {
                Process me = Process.GetCurrentProcess();
                foreach (Process process in Process.GetProcessesByName(me.ProcessName)) // Debug note: Set Enable the Visual Studio Hosting Process = false.
                {
                    if (process.Id != me.Id) // If the ProcessName matches but the Id doesn't, it's another instance of mine.
                    {
                        SetForegroundWindow(process.MainWindowHandle);
                        break;
                    }
                }
                Environment.Exit(0); // This will kill my instance.
            }
        }
    }
 
    // volatile is intended to prevent GC. Otherwise, GC.KeepAlive(appStartMutex) might be needed.
    // This appears to be a concern in In release builds but not debug builds.
    static volatile System.Threading.Mutex appStartMutex;
 
}