C#中抽象类

C#中方法的返回值类型是抽象类型：

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

namespace Delegate_Event
{
    class Program
    {
        static void Main(string[] args)
        {
 //返回抽象类类型,实际调用的子类的方法 
            var b = new C().getA("D");
            b.b();


            Console.WriteLine(b.b() + "||" + b.t());
        }
    }

    public abstract class A
    {
        public A() { }
        public virtual string t() { return "123A"; }
        public virtual string b() { return "123A"; }
    }

    public class B : A
    {
        public override string t()
        {
            return "123B";
        }

        public override string b()
        {
            return "123B";
        }
    }

    public class D : A
    {
        public override string t()
        {
            return "123D";
        }

        public override string b()
        {
            return "123D";
        }
    }

    public class C
    {
        public A getA(string type)
        {
            switch (type)
            {
                case "B":
                    return new B();
                case "D":
                    return new D();
                default:
                    return new B();
            }
        }
    }
}