Pass this to base constructor

Question

I am trying to implement good design patterns for a program I am writing. I have a class structure like this.

abstract class SomeBase
{
     public SomeObject obj { get; protected set; } 

     protected SomeBase(SomeObject x)
     {
           obj = x;
     }

     //Other methods and fields...
}

public class SomeDerived : SomeBase
{

     public SomeDerived() : base(new SomeObject(this))
     {

     }

}

Now as I'm sure you know, you can't pass this in a base constructor, because the object hasn't been initialized at this point in time. Anyhow I was really hoping there was a workaround. It's not best practice for me to allow SomeDerived() to handle the setting of a base classes field. I would like to pass this new object up the chain.

Answer 1

This is not possible, use an Init method after the constructor:

 abstract class SomeBase
 {
      private SomeObject _obj { get; set; } 
      public SomeObject obj 
      {
           get 
           {    // check _obj is inited:
                if (_obj == null) throw new <exception of your choice> ;
                return _obj;
           } 
      }

      protected SomeBase()
      {
           obj = null;
      }

      protected void Init()
      {
           obj = x;
      }

      //Other methods and fields...
 }

 public class SomeDerived : SomeBase
 {

      public SomeDerived() : base()
      {
           Init(new SomeObject(this));
      }

 }

Answer 2

Well, you actually have it in the base constructor, so there is no need to pass it.

using System;

abstract class SomeBase
{
    public SomeObject obj { get; protected set; }

    protected SomeBase()
    {
        // Will be executed as 1st step
        Console.WriteLine(1);

        // "this" here is SomeDerived object
        obj = new SomeObject((SomeDerived)this);
        // If you don`t like to depend upon SomeDerived type here,
        // you can do the same using the following line:
        //obj = (SomeObject)Activator.CreateInstance(typeof(SomeObject), this);
    }
}

class SomeObject
{
    public SomeObject(SomeDerived obj)
    {
        if (obj.obj == null)
        {
            // Will be executed as 2nd step
            Console.WriteLine(2);

            // You have the reference to SomeDerived here
            // But its properties are not yet initialized
            // (both SomeDerived and SomeBase constructors are in the progress)
            // So you should not access these properties 
            // in the SomeObject class constructor,
            // but you can do it in any method of SomeObject class 
            // (at the time the method will be called all constructors are finished).
        }
    }
}

class SomeDerived : SomeBase
{
    public SomeDerived()
    {
        // Will be executed as 3rd step
        Console.WriteLine(3);

        if (this.obj != null)
        {
            // You have the reference to SomeObject here,
            // which itself already got an access to SomeDerived reference
        }
    }
}

class MainClass 
{
    public static void Main (string[] args) 
    {
        var instance = new SomeDerived();
        Console.WriteLine (instance.obj); // Outputs SomeObject
    }
}

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Another solution will be to use the lambda expression, so the SomeDerived class will have total control on how the reference to it will be passed into SomeObject.

using System;

abstract class SomeBase
{
    public SomeObject obj { get; protected set; }

    protected SomeBase(Func<SomeBase, SomeObject> someObjectInitilizer)
    {
        // Will be executed as 1st step
        Console.WriteLine(1);

        // "this" here is SomeDerived object
        obj = someObjectInitilizer(this);
    }
}

class SomeObject
{
    public SomeObject(SomeDerived obj)
    {
        if (obj.obj == null)
        {
            // Will be executed as 2nd step
            Console.WriteLine(2);

            // You have the reference to SomeDerived here
            // But its properties are not yet initialized
            // (both SomeDerived and SomeBase constructors are in the progress)
            // So you should not access these properties 
            // in the SomeObject class constructor,
            // but you can do it in any method of SomeObject class 
            // (at the time the method will be called all constructors are finished).
        }
    }
}

class SomeDerived : SomeBase
{
    public SomeDerived() : base((@this) => new SomeObject((SomeDerived)@this))
    {
        // Will be executed as 3rd step
        Console.WriteLine(3);

        if (this.obj != null)
        {
            // You have the reference to SomeObject here,
            // which itself already got an access to SomeDerived reference
        }
    }
}

class MainClass 
{
    public static void Main (string[] args) 
    {
        var instance = new SomeDerived();
        Console.WriteLine (instance.obj); // Outputs SomeObject
    }
}

https://repl.it/@Konard/ParchedYellowgreenType