Code:

using System;
using SpecialServices;

namespace MyNameSpace
{
    public class MyForm : System.Windows.Forms.Form
    {

        private System.ComponentModel.Container components = null;

        public MyForm()
        {
            InitializeComponent();
            ...
        }

        [STAThread]
        static void Main() 
        {
            using(SingleProgramInstance spi = new SingleProgramInstance("MyForm"))
            {
                if (spi.IsSingleInstance)
                {

                    try 
                    {
                        Application.Run(new MyForm());
                    } 
                    catch  (Exception e) 
                    {
                        ...
                    }
                    Application.ExitThread();
                }
                else
                {
                    MessageBox.Show("Bla, bla",    "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
            }
        }
    }
    
    ...
}
    

Code:

[DllImport("kernel32.dll", EntryPoint="CreateMutexA")]
private static extern int CreateMutex (
    IntPtr lpMutexAttributes, 
    int bInitialOwner, 
    string lpName);

[DllImport("kernel32.dll")]
private static extern int GetLastError ();

private const int ERROR_ALREADY_EXISTS = 183;


public static bool IsInstanceRunning() {
      bool bIsRunning = false;
      try
      {
            string appname = Assembly.GetExecutingAssembly().GetName().Name;
            if ((CreateMutex(IntPtr.Zero, 1, appname ) != 0) && (GetLastError() == ERROR_ALREADY_EXISTS)) bIsRunning = true;
      }
      catch (Exception ex) {
            MessageBox.Show("Instance Error: " + ex.ToString());
      }
      return bIsRunning;
}


static void Main() {
    if(IsInstanceRunning()) Application.Exit();
    else Application.Run(new Form1());
}

Code:
using System.Threading;
using System.Diagnostics;

...

    static void Main() 
    {
        bool createdNew;
        Mutex singleInstanceMutex = new Mutex(true, Process.GetCurrentProcess().ProcessName, out createdNew);

        try
        {
            if (createdNew)
            {
                Application.Run(new Form1());
            }
        }
        finally
        {
            singleInstanceMutex.Close();
        }
    }