Create a constructor with parent class as parameter

Question

I don't know how to define my question (probably already asked but didn't found it).

I want to create a constructor for a class B inherited from A taking a B object as parameter used to be a copy of it.

There can be something like this :

class B : A
{
    public String NewField;

    public B(A baseItem, String value)
    {
        // Create new B to be a copy of baseItem
        ???; // something like : this = baseItem

        // Add new field
        NewField = value;
    }
}

Objective is to create an object B which is the exact copy of an A object with on filed more.

this is read-only within classes, add a copy constructor to A and call it from B's constructor. — Lee
– Lee, Commented Sep 13, 2017 at 12:59
learn.microsoft.com/en-us/dotnet/csharp/programming-guide/… — Eric Lippert
– Eric Lippert, Commented Sep 13, 2017 at 13:00
I know I can't assign this, I want to initiate the new B object to be a copy of the baseItem. — A.Pissicat
– A.Pissicat, Commented Sep 13, 2017 at 13:01
With A having a copy ctor, you'd do public B( A baseItem, string value) : base( baseItem ) { NewField = value;} — Fildor
– Fildor, Commented Sep 13, 2017 at 13:01

Izuka · Accepted Answer · 2017-09-13 13:09:23Z

3

Use the base keyword to call the parent class constructor, giving your parent class instance as a parameter. Then create a copy constructor in your parent, and you're done.

class A
{
    public A(A a)
    {
        // Copy your A class elements here
    }
}

class B : A
{
    public String NewField;

    public B(A baseItem, String value)
     : base(baseItem)
    {
        NewField = value;
    }
}

edited Sep 13, 2017 at 13:09

answered Sep 13, 2017 at 13:03

Izuka

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Jeroen van Langen · Accepted Answer · 2017-09-13 13:12:53Z

You could implement a CopyProperties method, which will copy the properties values.

using System;

public class A
{
    public string Filename {get; set;}

    public virtual void CopyProperties(object copy)
    {
        ((A)copy).Filename = this.Filename;
    }
}

public class B : A
{
    public int Number {get;set;}

    public override void CopyProperties(object copy)
    {
        base.CopyProperties(copy);
        ((B)copy).Number = this.Number;
    }    
}


public class Program
{

    public static void Main()
    {
        B b = new B { Filename = "readme.txt", Number = 42 };

        B copy = new B();
        b.CopyProperties(copy);

        Console.WriteLine(copy.Filename);
        Console.WriteLine(copy.Number);
    }
}

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