Question

May be I can’t explain exactly in words what I am trying to achieve, but I think this sample code can do:

class A
{
    public string Name
    {
        get;
        set;
    }
    public virtual void Say()
    {
        Console.WriteLine("I am A");
        Console.Read();
    }

    public bool ExtendA;

    public A GetObject()
    {
        if (ExtendA)
            return new B();

        return this;
    }
}

internal class B : A
{
    public override void Say()
    {
        Console.WriteLine(string.Format("I am {0}",Name));
        Console.Read();
    }
}

class Program
{
    static void Main(string[] args)
    {          
       var a = new A() {ExtendA = true,Name="MyName"};
        A ab = a.GetObject();            
    } 
}

As per the above code when the field Exitend A is set to true, and when again I try to get the object of same type from the same instance of object, I get the object but it loose the value of property ‘Name’.

Any suggestion how can I return back the class B with properties of A?

Thanks

Answer 1

I suggest you read a book or other resource on design patterns. For this you’d use a factory pattern.

You’d have your base class A and a class AFactory in addition to class B : A and a class BFactory.

At runtime you’d choose what you want to instantiate: A or B by using the factories:

IFactory factory = iWantClassA ? new AFactory() : new BFactory();
A a = factory.CreateInstance();

Although I agree with @ArnaudF in that I don’t see what you’re trying to accomplish. Why not just create subclass B directly?

UPDATE:

Having re-read your problem it sounds like you really just need a copy-constructor on your class, like so:

public class A {
    public A() {
        // Default constructor
    }
    public A(A other) {
        // Copy-constructor. Members of other will be copied into this instance.
    }
}

public class B : A {
    public B() {

    }
    public B(A other) : base(other) { // Notice how it calls the parent class's copy-constructor too
    }
}

Then to “upgrade” an instance of A to an instance of B at runtime, just use the copy constructor:

A a = new A();
// some logic here
B upgradedA = new B( a );