Program.cs

using System;
using System.Threading;
using System.Windows.Forms;

namespace TaskbarAutoHider
{
    internal static class Program
    {
        private static Mutex mutex = null;

        [STAThread]
        static void Main()
        {
            const string appName = "TaskbarAutoHider_SingleInstance";
            bool createdNew;

            mutex = new Mutex(true, appName, out createdNew);

            if (!createdNew)
            {
                MessageBox.Show("TaskbarAutoHider is already running.", "Already Running",
                    MessageBoxButtons.OK, MessageBoxIcon.Information);
                return;
            }

            try
            {
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new TrayApplicationContext());
            }
            finally
            {
                mutex?.ReleaseMutex();
                mutex?.Dispose();
            }
        }
    }
}