this is my first post to this group.... its about forcing your app to have only one instance running..

 

you need to import 2 dlls to find a window and to set focus to that window

/////////
[DllImport("User32",EntryPoint="FindWindow")]
private static extern IntPtr FindWindow(string lpClassName,string lpWindowName);

[DllImport("User32",EntryPoint="SetForegroundWindow")]
private static extern bool SetForegroundWindow(IntPtr hWnd);


[STAThread]
static void Main()
{
Process ThisProcess = Process.GetCurrentProcess();
Process [] AllProcesses = Process.GetProcessesByName(ThisProcess.ProcessName);                              

if (AllProcesses.Length > 1) // if the process is running
{
bool ret;          
IntPtr hWnd = FindWindow(null, "Form1");   //the caption of your window                 

if(hWnd != IntPtr.Zero)
    {
    ret = SetForegroundWindow(hWnd);  // give focus to this window

                //you can also use BringWindowToTop instead of this
     }                    
}

else
    {//run the new process

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