VS 2005.  When I google "CSharp single instance form" the returned pages
usually use a Mutex or the VB.NET runtime library.
  http://en.csharp-online.net/Application_Architecture_in_Windows_Forms_2.0%E2%80%
94Single-Instance_Detection_and_Management
  http://www.codeproject.com/csharp/SingleInstanceApplication.asp
  http://blogs.msdn.com/onoj/archive/2004/06/04/148532.aspx

"Program.cs" below seems to work fine.  A feature is it restores a minimized
form.  Is this code ok or is there a "gotcha"?  I'm aware it's probably
slower than a Mutex but restoring the minimized form is nice.

Thanks.

  -- Mark

========== Program.cs =========

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Diagnostics;
using System.Reflection;

namespace OneInstance {
   internal static class Program {
       /// <summary>
       /// The main entry point for the application.
       /// </summary>
       ///
       ///
       [STAThread]
       public static void Main() {
           //Get the running instance.
           Process instance = RunningInstance();

           if (instance == null) {
               //If no other instance of program is running, show form
               Application.Run(new Form());
           } else {
               //There is another instance of this process, do not allow
second
               HandleRunningInstance(instance);
               MessageBox.Show("Program already running! \n Focus
returned", "Already Open",
                               MessageBoxButtons.OK,
MessageBoxIcon.Exclamation);
           }
       }

       public static Process RunningInstance() {
           Process current = Process.GetCurrentProcess();
           Process[] processes =
Process.GetProcessesByName(current.ProcessName);

           //Loop through the running processes in with the same name
           foreach (Process process in processes) {
               //Ignore the current process if
               if (process.Id != current.Id) {
                   //Make sure that the process is running from the exe
file.
                   if
(Assembly.GetExecutingAssembly().Location.Replace("/", "\\")
                       == current.MainModule.FileName)
                       return process;
               }
           }
           return null;
       }

       public static void HandleRunningInstance(Process instance) {
           //Make sure the window is not minimized or maximized
           ShowWindowAsync(instance.MainWindowHandle, WS_SHOWNORMAL);

           //Set the real intance to foreground window
           SetForegroundWindow(instance.MainWindowHandle);
       }

       [DllImport("User32.dll")]
       private static extern bool ShowWindowAsync (IntPtr hWnd, int
cmdShow);

       [DllImport("User32.dll")]
       private static extern bool SetForegroundWindow(IntPtr hWnd);

       private const int WS_SHOWNORMAL = 1;
   }
}