Why can't I create an abstract constructor on an abstract C# class?

Question

I am creating an abstract class. I want each of my derived classes to be forced to implement a specific signature of constructor. As such, I did what I would have done has I wanted to force them to implement a method, I made an abstract one.

public abstract class A
{
    abstract A(int a, int b);
}

However I get a message saying the abstract modifier is invalid on this item. My goal was to force some code like this.

public class B : A
{
    public B(int a, int b) : base(a, b)
    {
        //Some other awesome code.
    }
}

This is all C# .NET code. Can anyone help me out?

Update 1

I wanted to add some things. What I ended up with was this.

private A() { }

protected A(int a, int b)
{
    //Code
}

That does what some folks are saying, default is private, and the class needs to implement a constructor. However that doesn't FORCE a constructor with the signature A(int a, int b).

public abstract class A
{
    protected abstract A(int a, int b)
    {


    }
}

Update 2

I should be clear, to work around this I made my default constructor private, and my other constructor protected. I am not really looking for a way to make my code work. I took care of that. I am looking to understand why C# does not let you do this.

Answer 1

Change that constructor in class A to

protected A(int a, int b)
{
    // Some initialisation code here
}

Then your subclasses will have to use it, as there is no default constructor.

They can, however, still change the actual signature of the constructor. There is no way of forcing a subclass to use a specific signature for its constructor as far as I know. I'm pretty sure constructors can't be abstract.

What exactly do you need this for? We might be able to suggest a work around for this.

Answer 2

You cannot enforce constructor signature, as each derived class may (must!) define its own constructor(s), and they may take any parameters they like.

If you need to pass a given set of variables to an object of a derived class, define an abstract method which needs to be implemented by derived classes. If the classes do not implement the abstract method, you will get a compiler error.

Answer 3

All subclasses always can specify their own constructor as long as they call a constructor of the superclass - so there is no way of forcing the a class to have a specific constructor (At least, that is the way it works in Java). You could see of using a factory pattern will get you somewhere - you could define a factory method in an interface - but you'll need a separate factory class as your abstract base class does not know the actual class of the object that needs to be created.

However: maybe adding a more concrete example of the issue you are having might prompt other/better responses. Are you looking for some generic instantiation code, or are you concerned specific settings on the abstract base class have to be done?

If you are just concerned about initialization that has to be done by the abstract class, create a method to do so, and document the usage of that method.

Answer 4

You cannot have an abstract constructor because abstract means you must override it in any non-abstract child class and you cannot override a constructor.

If you think about it, this makes sense, since you always call the constructor of the child class (with the new operator) and never the base class.

Generally speaking, the only way in C# to enforce a specific constructor signature is by using the new() generic constraint, which enforces the existence of a parameterless constructor for the type parameter.

Answer 5

not sure if this helps - but i feel this would be a solution to your problem:

public class A
{
  public A(int a, int b)
  {
    DoSomething(int a, int b);
  }

  virtual public void DoSomething(int a, int b)
  {

  }
}

public class B : A
{
  override public void DoSomething(int a, int b)
  {
    //class specific stuff
  }
}

with the result that you can call a constructor with the required arguments in any derived class with the correct behaviour.

Answer 6

Although you can't override constructors, and therefore can't define an abstract constructor, you can place an abstract factory method in your abstract base class. All the derived classes would need to override that.

public abstract class A 
{ 
    abstract A MakeAInstance(int a, int b); 
} 

public class B : A 
{ 
    // Must implement:
    override A MakeAInstance(int a, int b) {
        // Awesome way to create a B instance goes here
    }
}