抽象类没有任何抽象方法

Question

I am surprised to know that an abstract class in C# is possible with no abstract methods also.

abstract class AbstractDemo
{
 public void show()
  {
    Console.WriteLine("In Show Method"); 
   }
}
class MainDemo:AbstractDemo
{
 public static void Main()
 {
    Console.WriteLine("In Main Method");
 }
}

Any explaination ?

Answer 1

Sometimes you don't want to give the possibility to instantiate a class but you need this class as a base class for other classes.

The reason for choosing abstract classes over interfaces is that you can provide some basic implementation.

Answer 2

This is entirely valid, and occasionally useful if you want to provide event-like behaviour: provide an abstract class with all the "event handlers" implemented as virtual methods with a default behaviour of doing nothing.

A derived class can then override some of the methods, but doesn't have to override any specific one, because nothing's abstract. It still makes sense for the class to be abstract because an instance of the base class would be pointless (as everything would be a no-op).

This pattern is much more common in Java than C#, however - as in C# you'd normally just use "proper" events.

Answer 3

I think you're confusing abstract classes with interfaces. Interfaces can't have methods with body, abstract classes can. There are times when you want to prevent user from instantiating an object of a specific class; but still provide some base functionality for the classes that derive from it; this is what an abstract class is useful for.

Answer 4

An abstract class is a class that must be extended before it can be used. This does not it any way mean that the function themselves must be abstract.

Take for example an Animal class

public abstract class Animal
{
    void Move()
    {
        //whatever
    }
}

public class Fish : Animal
{
    void Swim()
    {

    }
}

public class Dog : Animal
{
    void Bark()
    {

    }
}

All animals can move but only the fish can swim and the dog can bark.

Or for a real life example. I have an Asp.net MVC base controller I use in my application. It has some basic methods I need very often like GetCurrentUser() and a function I wrote to help with localization. It also takes care of tracking so I don't have to rewrite that code in all of my controllers. The class has about 200 lines of code but not a single abstract method.

Answer 5

If your class is just a base for other classes and it does not have an full usablility - in other words as a base itselfe is not usable at all then you want to prevent from creating instances of it. In this case you can make abstract class without abstract members.

Answer 6

You could use abstract keyword on a class just to signal the compiler that it can only used inheriting from it, and not directly; In this case you are not oblied to put abstract member on the class.

This is equivalent to put in the class only one protected constructor, but using abstract is more clear and understandable.

Answer 7

No better explanation than MSDN it self http://msdn.microsoft.com/en-us/library/aa645615(v=VS.71).aspx

• An abstract class cannot be instantiated directly, and it is a compile-time error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.
• An abstract class is permitted (but not required) to contain abstract members.
• An abstract class cannot be sealed.

Answer 8

We have heard that in abstract class, there must be an abstarct member. But when I compile the abstarct class without an abstract method, it compiles. It gives me surprise. Now I am unable to find the article which explain exact behavior of an abstarct class.