openDecal/Managed/Decal.DenAgent/Program.cs

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

namespace Decal.DenAgent
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            // Single instance check
            using var mutex = new Mutex(true, "DecalAgentMutex", out bool createdNew);
            if (!createdNew)
            {
                MessageBox.Show("Decal Agent is already running.", "Decal Agent",
                    MessageBoxButtons.OK, MessageBoxIcon.Information);
                return;
            }

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