string strCurrentProcess = System.Diagnostics.Process.GetCurrentProcess().ProcessName;

I identify how many instances of the program are already running.

System.Diagnostics.Process.GetProcessesByName(strCurrentProcess).Length

If they are more than one, then a message is show to the user.

if (System.Diagnostics.Process.GetProcessesByName(strCurrentProcess).Length > 1)
{
Console.WriteLine("Another instance of this program is already running.");
...
}

The code itself is listed below.

using System;
    namespace ConsoleApplication1
    { 
    	class Class1 {
    		[STAThread]
    		static void Main(string[] args)
    		{
    			string strCurrentProcess = System.Diagnostics.Process.GetCurrentProcess().ProcessName;
    			if (System.Diagnostics.Process.GetProcessesByName(strCurrentProcess).Length > 1)
    			{
    				Console.WriteLine("Another instance of this program is already running.");
    				Console.ReadLine(); return;
    			}
    			else
    			{
    				Console.WriteLine("No another instance of this program is running.");
    				Console.ReadLine();
    			}
    		}
    	}
    }

Özgür Aytekin (MCSD .NET, MCDBA) www.dotnetcracks.com