Interface和abstract class

这两个概念在C#和Java类似，从语法上看

1. Interface和abstract class都不能实例化。


2. 可以继承多个Interface，但不能继承多个abstract class。


3. 继承Interface的子类必须实现Interface全部的方法，而abstract class子类可以选择是否实现基类的方法。


4. Interface除包含方法外，还可以包含属性，索引器，事件，不应该包含常量，构造函数，析构函数，静态成员。abstract
   class则可以包含上述任意成员。

从设计上看，Interface是定义继承的方法，而abstract class定义继承的对象。

　　1. Interface表达HAS-A的关系

　　2. abstract class表达IS-A的关系

How about an analogy: when I was in the Air Force, I went to pilot training
and became a USAF pilot. At that point I wasn‘t qualified to fly anything, and
had to attend aircraft type training. Once I qualified, I was a pilot
(Abstract class) and a C-141 pilot (concrete class). At one of my assignments,
I was given an additional duty: Safety Officer. Now I was still a pilot and a
C-141 pilot, but I also performed Safety Officer duties (I implemented
ISafetyOfficer, so to speak). A pilot wasn‘t required to be a safety officer,
other people could have done it as well.

All USAF pilots have to follow certain Air Force-wide regulations, and all
C-141 (or F-16, or T-38) pilots ‘are‘ USAF pilots. Anyone can be a safety
officer. So, to summarize:

• Pilot: abstract class


• C-141 Pilot: concrete class


• ISafety Officer: interface

added note: this was meant to be an analogy to help explain the concept,
not a coding recommendation. See the various comments below, the discussion is
interesting.　　

reference：

http://kb.cnblogs.com/page/41836/

http://dev.yesky.com/436/7581936.shtml

http://stackoverflow.com/questions/761194/interface-vs-abstract-class-general-oo?lq=1

http://stackoverflow.com/questions/56867/interface-vs-base-class