定义父类为虚方法时需要关键字  virtual

子类重写需要关键字  override

示例代码：

 //父类
   public class People
    {
        //虚方法
        //VR Virtual Reality  虚拟现实
        //AR Augmented Reality 增强现实
        public virtual void SayHello()
        {
            Console.WriteLine("父类打招呼的方法");
        }

}

 //子类（重写父类方法）
  public  class Chinese:People
    {
      public override void SayHello()
      {
          Console.WriteLine("你好！");
      }
    }

    //子类
    public class American:People
    {
        public override void SayHello()
        {
            Console.WriteLine("Hello");
        }
    }

 public  static void Main(string[] args)
        {
          //统一调用
            List<People> list = new List<People>()
           {
            new Chinese(),
            new American()
           };

            People people = new People();
            people.SayHello();
            foreach (People item in list)
            {
                item.SayHello();
            }
            Console.ReadKey();
        }

继承（父类为虚方法以及子类的重写）