the Differences between abstract class & interface in C#介面和抽象類別的區別

    abstract class and interface in C# look like much, but sometimes they get the similiar use. However, there are still much differences between them. here we go.
    抽象類別和介面初看上去很象，但是還是有很多區別的。
   a) Interface Grammar介面文法
   [public|protected|internal|private] interface interfaceName
    {
        public int add(int x,int y);
//this is error! interface memeber can not be described with any characters to declare its access attribute.
//介面中任何成員和屬性不能被任何修飾符修飾來表明其訪問屬性！
        int add(int x,int y);
//this is right!
        int Count{get;} //interface property介面屬性
        string this[int index]{get;set;}//index interface 介面索引
    }

    Difference:    
        1. in interface, all interface members can not be decorated with any other access tag,such as public,private.etc.
        2. in abtract class, such case is just on the opposite.
    
    b) abstract class Grammar 抽象類別文法
        [public|protected|internal|private] abstct class className
        {
            public abstract int add(int x,int y);
            //it's ok, but that forbidden in interface!
            //在abstract class中，類成員可以有各種修飾符。如果該類成員沒有被implement,那麼該類一定要被標示為abstact方法。否則出錯！當然也可以實現該方法。如下multiply方法所示
            public virtual int multiply(int x,int y)
            {
                return x*y;
            }
        }

        Difference:
        1.in interface, all interface memebers can not be implemented. and it's just an declaration  for other class which will inherented from it and implemented all of them.
        在介面中，所有介面成員都不能被實現，它只是一個申明而已，其他類繼承自該介面，並且要實現該介面中的所有成員。
        2 in abstract class, its member can be implemented ,otherwise must be decorated with abstract that is the same with interface . of course, any class inherented from this  must implemented all its abstract method. of course, u can also override the method that has already implemented method.
        在抽象類別中，其成員可以被實現，如果不實現的話，其方法成員一定要被標標示為abstract. 同時，任何繼承於該抽象類別的類必須要實現其所有標示為abstract的方法，這點與interface相同。或者該類也可以override該抽象類別中的method. 這一點interface絕對不可能,因為interface中根本沒有實現過，何來override?