1    // Detect existing instances

2    string processName = Process.GetCurrentProcess().ProcessName;
3    Process[] instances = Process.GetProcessesByName(processName);
4
5    if (instances.Length > 1)
6    {
7        MessageBox.Show("This application is already running.");
8        return;
9    }
10    // End of detection

static void Main()
{
    // Detect existing instances
    string processName = Process.GetCurrentProcess().ProcessName;
    Process[] instances = Process.GetProcessesByName(processName);

    if (instances.Length > 1)
    {
        MessageBox.Show("This application is already running.");
        return;
    }
    // End of detection

    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Application.Run(new Form1());
}