新人报道 对象作为值传递疑惑

namespace 引用类型对象作为值参数传递_看书不能理解_
{
    class Myss {

        public int Myss1 = 20;

}
    class Program
    {
        static void Test(Myss f1) {
            f1.Myss1 = 50;
            Console.WriteLine($"After member assigment; {f1.Myss1}");
            f1 = new Myss();
            Console.WriteLine($"After new object creation {f1.Myss1}");

        }

        static void Main(string[] args)
        {
            Myss a1 = new Myss();
            Console.WriteLine($"Before method call; {a1.Myss1}");
            Test(a1);
            Console.WriteLine($"afore method call; {a1.Myss1}");
            Console.ReadKey();
        }
    }
}
为什么a.Myss1==50