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

 public class OneInstnace
 {
 [STAThread]
 public static void Main()
 {
 //Get the running instance.
 Process instance = RunningInstance();
 if (instance == null)
 {
 //There isn't another instance, show our form.
 Application.Run (new Form());
 }
 else
 {
 //There is another instance of this process.
 HandleRunningInstance(instance);
 }
 }
 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 (process.Id != current.Id)
 {
 //Make sure that the process is running from the exe file.
 if (Assembly.GetExecutingAssembly().Location.Replace("/", "\\") ==
 current.MainModule.FileName)
 {
 //Return the other process instance.
 return process;
 }
 }
 }

 //No other instance was found, return null.
 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;
 }