C#

[STAThread]
        static void Main()
        {
            //Get the running instance.
            Process instance = RunningInstance();
            if (instance == null)
            {
                //There isn't another instance, show our form.

                    Application.Run(new AteGui(splashScrn));            
                
            }
            else
            {
                //There is another instance of this process.
                HandleRunningInstance(instance);
            }
        }

<big>Helper Functions and DLL imports</big>
 /// <summary>
        /// Property to set the visible status of the Tutorial Button
        /// </summary>
        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;

toolbar_onclick(...)
{
  if(e.Button == toolbarButton1)
    button1.PerformClick();
}