better.cs

using MessagePack;
using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Threading;

namespace ConsoleApp
{
    public interface IS
    {
        bool ShouldDoStuff { get; set; }
        void MethodWithGeneric<T>();
    }

    public interface MyService
    {
        void AddPart(Action<IS> action);
    }

    public class SomeType { }
    public class SomeOtherType { }


    public static class Program
    {

        static unsafe void Main()
        {
            MyService myService = null;

            myService.AddPart(s =>
            {
                s.ShouldDoStuff = true;
                s.MethodWithGeneric<SomeType>();
            });
            myService.AddPart(s =>
            {
                s.ShouldDoStuff = false;
                s.MethodWithGeneric<SomeOtherType>();
            });

            myService.AddPart<SomeType>(true);
            myService.AddPart<SomeOtherType>(false);

        }

        public static void AddPart<T>(this MyService myService, bool shouldDoStuff )
        {
            myService.AddPart(s =>
            {
                s.ShouldDoStuff = shouldDoStuff;
                s.MethodWithGeneric<T>();
            });
        }
    }
}