Execute a derived constructor before the base constructor in C#

Question

My problem here is that I would like to pass an object to a derived class, but it must be done before the base class constructor, since the base class will immediately call the derived class's Start() method that uses the object.

Here's an excerpt from the base class, (renamed from BarcodeScanner for convenience).

public abstract class MyBase
{    
    public MyBase()
    {
        if (Initialize())
            this.Start();
    }

    public abstract bool Initialize();
    public abstract void Start();
}

Here's the derived class that I'm creating.

class MyDerived : MyBase
{
    private string sampleObject;

    public MyDerived (string initObject)
    {
        sampleObject = initObject;
    }

    public override bool Initialize() 
    { 
        return GetDevice();
    }
    public override void Start() 
    { 
        Console.WriteLine("Processing " + sampleObject.ToString()); 
    }
}

I doubt you can make C# execute a derived constructor before the base constructor; so I'm really just looking for a solution to pass an object to the derived class before the object is used.

I've gotten around this by putting the Initialize/Start if block inside the MyDerived constructor. However, there are other classes deriving from the base class; so I ended up having to repeat this block of Initialize/Start code in every derived class. I'd like to see an alternative to modifying the base class.

Answer 1

What you are trying to do is impossible in C#. A constructor in a base class must be run before the constructor of any derived class otherwise there would be potential for corrupt object state. A child object must be able to assume that its base is fully constructed and available.

Answer 2

IMHO your design is wrong. You shouldn't start the process from within the constructor. Your consuming code should explicitly call the Start() method when required.

Answer 3

I would rework your design so that Initialize (and potentially Start() - though I'd normally have this be a public method that's called by the user) are called after construction.

If you're making a BarcodeScanner, you could do this the first time you go to scan. Just lazy-initialize your members using the data from the derived class.

This will work around your issue, with no real change in usage from the user.