Force an inheritor to initialise a field of the base from its CTOR (C#)

Question

I'm trying to enforce an inheritor of an abstract class to initialize one of the fields in the base.

I can't do this via a constructor in the base because the type of the field requires a reference back to the object creating it. (I suppose I could provide a parameterless ctor for the field type. But I feel that would just be shifting the problem, and it should be immutable anyway).

Perhaps someone can suggest how to do it, or maybe make it so I don't need to do it at all?

This is pretty much what I'm doing:

public abstract class Base
{
    // Would like to force setting in CTOR
    protected BaseImplementation _implementation;  
    protected BaseImplementation Implementation 
    { get { return _implementation; } }

    public void DoSomething()
    {    
        Implementation.DoSomething();
    }
}

public abstract class BaseImplementation
{
    public abstract void DoSomething();
}

public class MyObject : Base
{
    // Note its Nested
    public class MyImplementation : BaseImplementation
    {
        private MyObject _myObject;

        public MyImplementation(MyObject myObject)
        {
            this._myObject = myObject;
        }

        public override void DoSomething()
        {
            // Does something
        }
        public virtual void SomethingElse()
        {
            // Does something else 
        }
    }

    new public MyImplementation Implementation
    {
        get { return (MyImplementation) _implementation; }
    }

    public void SomethingElse()
    {
        Implementation.SomethingElse();
    }
}

To give a little context, I have a number of classes which inherit from the base. They, intern have multiple ways which they can manipulate their state. This has been the best solution I have come up with for handling that.

Edit

I forgot to mention that I'm not using an Abstract property because it means I cant use new public MyImplementation Implementation {.... I don't want to have to cast Implementation to MyImplementation whenever I use functionality specific to the derived class (thus using new in the first place) or create multiple fields containing the same object.

Answer 1

Don't add a field to the base class. Simply make it an abstract property. Derived classes will have to override it and return their own value.

protected abstract BaseImplementation Implementation { get; }

Alternatively, you can make the base class constructor accept an argument of type BaseImplementation and have the derived class initialize it:

public abstract class Base { 
    protected Base(BaseImplementation implementation) {
        _implementation = implementation; 
    }

    private BaseImplementation _implementation;  
    protected BaseImplementation Implementation { get { return _implementation; } }
}

public class Derived : Base {
    public Derived() : base(new MyImplementation()) { }
    // ...
}