Set process to be critical

criticalProc.cs

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;

namespace UnkillableTest
{
    class Program
    {
        [DllImport("ntdll.dll", SetLastError = true)]
        private static extern void RtlSetProcessIsCritical(uint bNew, uint pbOld, uint bNeedScb);
        
        public static void MakeProcessUnkillable()
        {
            Process.EnterDebugMode();
            RtlSetProcessIsCritical(1, 0, 0);
        }
        
        public static void MakeProcessKillable()
        {
            RtlSetProcessIsCritical(0, 0, 0);
        }
        
        static void Main()
        {
            MakeProcessUnkillable();
            Environment.Exit(0);
        }
    }
}