<span style="font-size:18px;">using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
 
namespace qq 
{ 
  public abstract class A//抽象类 
  { 
    public abstract void add(int a, int b);//抽象方法 
  } 
  public class B : A//派生类B 
  { 
    public override void add(int a, int b)//重写抽象方法 
    { 
      int sum = a + b; 
      Console.WriteLine(sum); 
    } 
  } 
  public class C : A//派生类C 
  { 
    public override void add(int a, int b)//重写抽象方法 
    { 
      int sum = System.Math.Abs(a + b);//取两个数的和的绝对值 
      Console.WriteLine(sum); 
    } 
  } 
  class Program 
  { 
    static void Main(string[] args) 
    { 
      B p = new B(); 
      p.add(1,-2); 
      C q = new C(); 
      q.add(1,-2); 
      Console.ReadLine(); 
    } 
  } 
} 
</span> 

<span style="font-size:18px;">using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
 
namespace qq 
{ 
  public class abtext 
  { 
    public virtual void output() 
    { 
      Console.WriteLine("这是虚方法"); 
    } 
  } 
  abstract public class class1 : abtext 
  { 
    public abstract override void output(); 
  } 
  public class class2 : class1 
  { 
    public override void output() 
    { 
      Console.WriteLine("这是一个新的方法"); 
    } 
  } 
  class Program 
  { 
    static void Main(string[] args) 
    { 
      class2 c1 = new class2(); 
      c1.output(); 
      Console.ReadLine(); 
    } 
  } 
} 
</span>