the Differences between abstract class & interface in C#接口和抽象类的区别

    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.
    抽象类和接口初看上去很象，但是还是有很多区别的。
   a) Interface Grammar接口语法
   [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.
//接口中任何成员和属性不能被任何修饰符修饰来表明其访问属性！
        int add(int x,int y);
//this is right!
        int Count{get;} //interface property接口属性
        string this[int index]{get;set;}//index interface 接口索引
    }

    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 抽象类语法
        [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，类成员可以有各种修饰符。如果该类成员没有被implement,那么该类一定要被标示为abstact方法。否则出错！当然也可以实现该方法。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.
        在接口中，所有接口成员都不能被实现，它只是一个申明而已，其他类继承自该接口，并且要实现该接口中的所有成员。
        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.
        在抽象类中，其成员可以被实现，如果不实现的话，其方法成员一定要被标标示为abstract. 同时，任何继承于该抽象类的类必须要实现其所有标示为abstract的方法，这点与interface相同。或者该类也可以override该抽象类中的method. 这一点interface绝对不可能,因为interface中根本没有实现过，何来override?