C＃中的抽象类构造函数

Question

In c# we can't create an obeject of a abstact class or interface it means abstract class do not have any constructor, is it true ? or if it have then what is it's purpose there?

Answer 1

As others have said, abstract classes usually have constructors (either explicitly or the default one created by the compiler) - and any derived class constructor will have to chain through the abstract class's constructor in the normal way. That's the important bit... suppose you have an abstract class which stores the name associated with an instance - because you always want a name, and you don't want to write the Name property in each concrete derived class. You might provide a constructor which takes that name and assigns it to a field... and then every subclass constructor would have to go through that constructor, so that you still knew you'd always have a name. If you want to know more about constructor chaining, read my article on it .

Here's an example of that:

public abstract class DemoBase
{
    private readonly string name;    
    public string Name { get { return name; } }

    protected DemoBase(string name)
    {
        this.name = name;
    }

    // Abstract members here, probably
}

public class FixedNameDemo : DemoBase
{
    public FixedNameDemo()
        : base ("Always the same name")
    {
    }

    // Other stuff here
}

public class VariableNameDemo : DemoBase
{
    public VariableNameDemo(string name)
        : base(name)
    {
    }

    // Other stuff here
}

To further answer your comment on BoltClock's answer, asbtract classes can't have private abstract methods, but they can have private constructors. Indeed, it's sometimes useful to have only private constructors in an abstract class, because it means the class can only be derived from within the program text of the same class . This allows you to create pseudo-enums:

public abstract class ArithmeticOperator
{
    public static readonly ArithmeticOperator Plus = new PlusOperator();
    public static readonly ArithmeticOperator Minus = new MinusOperator();

    public abstract int Apply(int x, int y);

    private ArithmeticOperator() {}

    private class PlusOperator : ArithmeticOperator
    {
        public override int Apply(int x, int y)
        {
            return x + y;
        }
    }

    private class MinusOperator : ArithmeticOperator
    {
        public override int Apply(int x, int y)
        {
            return x - y;
        }
    }
}

In this respect, an abstract private method/property could make sense - it could be accessed by the base class but provided by the derived classes within the same class's program text. However, it's prohibited by the specification. Usually, protected abstract members would solve the same problem - but not quite always.

Answer 2

Good question. Here's why Abstract classes need constructors even though they cannot be instantited.

In any Object oriented language like C#, object construction is an hierarchical process. Look at the code below. When you instantiate any object of type DerivedClass, it must construct the base object first before creating the object of typeof DerivedClass. Here the base class may or may not be an Abstract class. But even when you instantiate an object of a concrete type derived from an abstract class it will still need to call the constructor of the Base class before the object of DerivedClass type is created, hence you always need a constructor for Abstract class. If you have not added any constructor, C# compiler will automatically add a public parameterless constructor to the class in the generated MSIL.

public class BaseClass
{
    public BaseClass() 
    {
        Console.WriteLine("BaseClass constructor called..");
    }
}

public class DerivedClass : BaseClass
{
    public DerivedClass()
    {
        Console.WriteLine("DerivedClass constructor called..");
    }
}

DerivedClass obj = new DerivedClass();

//Output
//BaseClass constructor called..
//DerivedClass constructor called..

PS : Assuming, If Abstract base classes are not allowed to have constructors because they need not be instantiated, the whole fundamentals of the object oriented programming will go on toss. The idea behind Abstract types are to represent objects that have some features and behaviours but not complete as whole to allow independant existence.

Answer 3

No. it means that operator new is not allowed to create object from this type of class.

The purpose might be that are allocated/initialized some properties of class.

abstract usually leave some methods to implement.

Regarding the interface, this structure holds only the signatures of method, delegates or events. That may be implemented in class that use interface. You cant create a object.

Read about new

EDIT:

What is the purpose of constructor in abstract class ?

When one class inherit another class, the parent class of it had to be created first while object is crated. In class do not implement some special constructor always is used default one [className()]. When you override some method then the implementation of functionality is taken form class which override the method. This is why method used in constructor should never be virtual. Same logic for abstract class, such class can have a lot of functionality, and only one method that should be implemented by child class.

Answer 4

Abstract classes have constructors but you can't call them directly as you can't directly instantiate abstract classes.

To answer your comment, the concept of a private abstract method or property makes no sense, because private prevents anybody else from accessing it, and abstract prevents itself from accessing it. So there would essentially be no possible way to call it.

EDIT: see Jon Skeet's answer on private constructors. Private members of other kinds cannot exist in abstract classes, though.

Answer 5

Abstract classes do have constructors. When you create an instance of a derived class, its parent class' constructors are called. This applies to classes derived from abstract classes as well.