Lazy singleton for service with default implementation

LazySingletonForServiceWithDefaultmplementation.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp7
{
    public interface IService
    {
        string Foo();
        string Bar(string a);
    }
    
    //Static class is a must: "A namespace cannot directly contain members such as fields or methods" :(
    public static class Dummy// : IService //static class can not implement interfaces :(
    {
        private static IService _service = null;
        public static IService Singleton
        {
            get
            {
                return _service ?? new BaseImplementation();
            }
            set
            {
                _service = value;
            }
        }

        public static string Bar(string a)
        {
            return Singleton.Bar(a);
        }

        public static string Foo()
        {
            return Singleton.Foo();
        }
    }

    public class BaseImplementation : IService
    {
        public BaseImplementation()
        {
            Dummy.Singleton = this;
        }

        public virtual string Bar(string a)
        {
            return a.ToUpper();
        }

        public virtual string Foo()
        {
            return this.GetType().ToString();
        }
    }

    public class MyImplementation : BaseImplementation
    {
        public override string Bar(string a)
        {
            return a.ToLower();
        }

        public override string Foo()
        {
            return this.GetType().ToString();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(Dummy.Foo());
            Console.WriteLine(Dummy.Bar("AaA"));

            new MyImplementation();

            Console.WriteLine(Dummy.Foo());
            Console.WriteLine(Dummy.Bar("AaA"));

            Console.ReadLine();
        }
    }
}