C＃中的单例实现问题

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

namespace ConsoleApplication2
{
    public class MsgService
    {
        private static CreateConnectionToA _instanceA;
        private static CreateConnectionToB _instanceB;

        protected MsgService()
        {

        }

        public static MsgService GetInstanceA(string paramA, string paramB)
        {

            if (_instanceA != null)
            {
                return _instanceA;
            }

            return _instanceA = new CreateConnectionToA("p1","p2");
        }


        public static MsgService GetInstanceB(string paramA, string paramB)
        {

            if (_instanceB != null)
            {
                return _instanceB;
            }

            return _instanceB = new CreateConnectionToB("p1", "p2");
        }


    }




}


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

namespace ConsoleApplication2
{
    class CreateConnectionToB : MsgService
    {
        public CreateConnectionToB(string param1, string Param2)
        {

        }
    }
}


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

namespace ConsoleApplication2
{
    class CreateConnectionToA : MsgService
    {
        public CreateConnectionToA(string param1, string Param2)
        {

        }

    }
}


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

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {

            MsgService.GetInstanceA("p1", "p2");

            Console.Read();
        }
    }
}

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Press");
            Console.Read();

            ConsoleApplication2.MsgService.GetInstanceA("p1", "p2");
            Console.Read();
        }
    }
}