Interface和abstract class

這兩個概念在C#和Java相似，從語法上看

1. Interface和abstract class都不能實例化。
2. 能夠繼承多個Interface，但不能繼承多個abstract class。
3. 繼承Interface的子類必須實現Interface所有的方法，而abstract class子類能夠選擇是否實現基類的方法。
4. Interface除包含方法外，還能夠包含屬性，索引器，事件，不該該包含常量，構造函數，析構函數，靜態成員。abstract class則能夠包含上述任意成員。

從設計上看，Interface是定義繼承的方法，而abstract class定義繼承的對象。

　　1. Interface表達HAS-A的關係

　　2. abstract class表達IS-A的關係

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