using System;
namespace ConsoleApplication8 
{ 
class Program 
{ 
static void Main(string[] args) 
{ 
BClass b = new BClass(); 
b.m1(); 
} 
} 
abstract class AClass 
{ 
public abstract void m1(); 
public abstract void m2(); 
} 
class BClass : AClass 
{ 
public override void m1() 
{ 
throw new NotImplementedException(); 
} 
//public override void m2() 
//{ 
// throw new NotImplementedException(); 
//} 
} 
} 

interface I 
{ 
void M(); 
} 
abstract class C: I 
{ 
public abstract void M(); 
} 
抽象類實例: 
// abstract_keyword.cs 
// 抽象類 
using System; 
abstract class BaseClass // 抽象類 
{ 
protected int _x = 100; //抽象類可以定義字段,但不可以是抽象字段,也沒有這一說法. 
protected int _y = 150; 
public BaseClass(int i) //可以定義實例構(gòu)造函數(shù),僅供派生的非抽象類調(diào)用; 這里顯式提供構(gòu)造函數(shù),編譯器將不再提供默認構(gòu)造函數(shù). 
{ 
fielda = i; 
} 
public BaseClass() 
{ 
} 
private int fielda; 
public static int fieldsa = 0; 
public abstract void AbstractMethod(); // 抽象方法 
public abstract int X { get; } //抽象屬性 
public abstract int Y { get; } 
public abstract string IdxString { get; set; } //抽象屬性 
public abstract char this[int i] { get; } //抽象索引器 
}
class DerivedClass : BaseClass 
{ 
private string idxstring; 
private int fieldb; 
//如果基類中沒有定義無參構(gòu)造函數(shù),但存在有參數(shù)的構(gòu)造函數(shù), 
//那么這里派生類得構(gòu)造函數(shù)必須調(diào)用基類的有參數(shù)構(gòu)造函數(shù),否則編譯出錯 
public DerivedClass(int p) 
: base(p) //這里的:base(p)可省略,因為基類定義了默認的無參構(gòu)造函數(shù) 
{ 
fieldb = p; 
} 
public override string IdxString //覆蓋重新屬性 
{ 
get 
{ 
return idxstring; 
} 
set 
{ 
idxstring = value; 
} 
} 
public override char this[int i] //覆蓋重寫索引器 
{ 
get { return IdxString[i]; } 
} 
public override void AbstractMethod() 
{ 
_x++; 
_y++; 
}
public override int X // 覆蓋重寫屬性
{
 get
 {
  return _x + 10;
 }
}

public override int Y // 覆蓋重寫屬性
{
 get
 {
  return _y + 10;
 }
}

static void Main()
{
 DerivedClass o = new DerivedClass(1);
 o.AbstractMethod();
 Console.WriteLine("x = {0}, y = {1}", o.X, o.Y);
}
}