If you only want 1 (one) instance of your C# application running, then simply make use of the following code in the Form1 (or whatever you form is called) constructor, just after InitializeComponent();
if (Process.GetProcessesByName(Process.GetCurrentProcess().ProcessName).Length > 1)
MessageBox.Show(“Only one instance allowed”);
Environment.Exit(0); //exits the application
//load windows forms application if this is the only instance
Have fun and happy coding!