Call child implementation of abstract class type C#

Question

I want to call the child class implementation on an object with the abstract class type. However, this doesn't work the way I thought it would. Is there a way to do it, that doesn't require me to switch between types in the second switch satement? Or does C# not allow for this type of behavior?

The code where it is called:

AbstractParentType wfp;

//Switch on diagram type and select processor
switch (qi.DIAGRAMTYPE)
{
    case 1:
        wfp = new T1(notifications);
        break;
    case 2:
        wfp = new T2(notifications);
        break;
    case 3:
        wfp = new T3(notifications);
        break;
    default:
        throw new Exception("Diagramtype not implemented");
}

bool result = false;
//Switch on action type
switch (qi.Type)
{
    case (int)WorkflowActionType.BelItem:
        //Do some case specific stuff here
        ...
        //Call method
        result = wfp.Meth1();
        break;
    ... (a bunch of cases) ...
    case (int)WorkflowActionType.WordDocument:
        //Do some case specific stuff here
        ...
        //Call method
        result = wfp.Meth10();
        break;
}

Then we have the classes implementations:

abstract class AbstractClassType {
     public bool Meth1() { ... }
     ...
     public bool Meth10() { ... }
     ...
     public abstract MethX();
     public abstract MethY();
}

class T1 : AbstractClassType  {
     public new Meth1() { ... }
     ...
     public new Meth10() { ... } 
     ...
     public override MethX() { ... }
     public override MethY() { ... }
}

The actual methods do have parameters and I do want a base implementation for some of the methods (but not all of them). The goal is to allow the inherting classes to 'extend' the methods behavior.

Answer 1

Try using the virtual keyword

When using virtual, you can give methods in the base class a 'default' implementation. Like so:

abstract class AbstractClassType {
    public virtual void MethX(){
        //default implementation here.            
    }
    public virtual void MethY(){
        //another default implementation here!
    }
}

class T1 : AbstractClassType {
    public override void MethX(){
        //base.MethX() would call the logic in the base class. 
    }
    public override void MethY(){ 
        //base.MethY() would call the logic in the base class. 
    }
}

The difference between virtual and abstract is that basically, an abstract method cannot have a base implementation, and must be overridden.

A virtual method can have a base implementation, and does not need to be overriden.

You're not required to call base.MethX/Y() . You could even give the method a whole new meaning if you wanted to.

Answer 2

First of all, you cannot create an object of an abstract class as it is not really a complete entity. You will always need to instantiate the object of a class that extends the abstract class.

Following code shows various, not all, options you have when working with abstract classes.

    public abstract class AbstractClass
    {
        public void OnlyInAbstract() {
            Console.WriteLine("You are stuck with OnlyInAbstract in abstract class unless you use new keyword.");
        }

        public virtual void OnlyInAbstractForNow()
        {
            Console.WriteLine("You have reached abstract class for now. However, override me for changed behaviour.");
        }

        public abstract void MustImplement();
    }

    public class FirstChild : AbstractClass
    {
        public override void MustImplement()
        {
            Console.WriteLine("You called MustImplement in FirstChild. Nothing else to see here.");
        }

        public override void OnlyInAbstractForNow()
        {
            base.OnlyInAbstractForNow();
            Console.WriteLine("I see you changed my behaviour in FirstChild to extend it after abstract class was done with.");
        }

        public new void OnlyInAbstract()
        {
            Console.WriteLine("Looks like we are making an all new OnlyInAbstract method in child class.");
        }
    }

    static void Main(string[] args)
    {
        AbstractClass abstractClass = new FirstChild();

        abstractClass.MustImplement();

        abstractClass.OnlyInAbstract();

        (abstractClass as FirstChild).OnlyInAbstract();

        abstractClass.OnlyInAbstractForNow();

        Console.Read();
    }