C#中对虚拟属性和抽象属性的重写有什么异同

       public abstract class A
        {
            //抽象属性不能有实现代码
            public abstract string AbstractProperty { get; set; }
 
            string s;
            //虚拟属性可以有实现代码
            public virtual string VritualProperty
            {
                get { return this.s; }
                set { this.s = value.ToUpper(); }
            }
        }
 
        public class B : A
        {
            string message;
            //在继承类（子类）中必须提供抽象属性的实现
            public override string AbstractProperty
            {
                get { return message; }
                set { this.message = value; }
            }
 
            //重写属性可以调用基类中的实现
            public override string VritualProperty
            {
                get
                {
                    return base.VritualProperty;
                }
                set
                {
                    base.VritualProperty = value;
                }
            }
        }