C# [method Modifiers] abstract virtual override new

abstract :表示方法是抽象方法，在子类中必须重写。抽象方法所在的类必须是抽象类，即用abstract modifiers；
virtual:表示此方法是virtual方法，除了在子类中可以重写外（在子类中也可直接使用），和普通方法完全一样；
override:表示重写父类的virtual方法；
new： 显式隐藏从基类继承的成员；

区别：

virtual:标记方法为虚方法
1.可在派生类中以override覆盖此方法
2.不覆盖也可由对象调用
3.无此标记的方法(也无其他标记)，重写时需用new隐藏原方法
abstract 与virtual : 方法重写时都使用 override 关键字

Eg1:

   public abstract class Book
    {
        public Book()
        {
        }
        public abstract void getPrice(); //抽象方法，不含主体
        public virtual void getName() //虚方法，可覆盖
        {
            Console.WriteLine("this is a test:virtual getName()");
        }
        public virtual void getContent() //虚方法，可覆盖
        {
            Console.WriteLine("this is a test:virtual getContent()");
        }
        public void getDate() //一般方法，若在派生类中重写，须使用new关键字
        {
            Console.WriteLine("this is a test: void getDate()");
        }
    }

    public class ChineseBook : Book
    {
        public override void getPrice() //实现抽象方法，必须实现
        {
            Console.WriteLine("this is a test:ChineseBook override abstract getPrice()");
        }
        public override void getName() //覆盖原方法，不是必须的
        {
            Console.WriteLine("this is a test:ChineseBook override virtual getName()");
        }
        public new void getDate() {
            Console.WriteLine("this is a test:ChineseBook new getDate()");
        }

        public void Run() {
            getPrice();
            getName();
            getContent();
            getDate();
        }

    }

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

Output:

this is a test:ChineseBook override abstract getPrice()
this is a test:ChineseBook override virtual getName()
this is a test:virtual getContent()
this is a test:ChineseBook new getDate()

Eg2:

 public abstract class FlowModel
    {
       public abstract void A();

       public virtual void B(){
           Console.WriteLine("Orginal B()");
       }

       public virtual void C(){
           Console.WriteLine("Orginal C()");
       }

       public void D(){
           Console.WriteLine("Orginal D()");
       }

    }

   public class Flow:FlowModel
   {
       public override void A()
       {
           //执行步骤A
           Console.WriteLine("Execute Step A ");
       }

       public virtual void B()
       {
           //执行步骤B
           Console.WriteLine("Execute Step B");
       }

       public void C()
       {
           //执行步骤C
           Console.WriteLine("Execute Step C");
       }

       public new void D() {
           Console.WriteLine("Execute Step D");
       }

       public void Run()
       {
           A();
           B(); //步骤B是扩展点　，可以由子类决定具体执行什么
           C();
           D();
       }
   }

Output:

Execute Step A
Execute Step B
Execute Step C
Execute Step D

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }