有关C#中的引用类型的内存问题

 public class ObjectRef
    {
        public static void Demo_Main()
        {
            PointD a, b, c;

            b = new PointD(100, 58);
            a = b;
            b = new PointD(200, 11);
           

            Console.WriteLine("a is now {0}, {1}", a.X, a.Y); // a is now 100, 58
            c = new PointD(300, 22);
            a = c;
            c.X = 500;
            Console.WriteLine("a is now {0}, {1}", a.X, a.Y); // a is now 500, 22
        }
    }


    public class PointD
    {
        int x;
        int y;

        public int X
        {
            get { return x; }
            set { x = value; }
        }
        
        public int Y
        {
            get { return y; }
            set { y = value; }
        }

        public PointD(int x, int y)
        {
            this.x = x;
            this.y = y;
        }
    }