Web Dashka

A few days ago I have found out for myself about one more difference between PHP and C# OOP impelentations. Look at the following code snippet in PHP:

class ParentClass {
    public $val = 'Parent';

    public function GetValue()
    {
        echo $this->val;
    }
}

class ChildClass extends ParentClass {
     public $val = 'Child';
}

$a = new ChildClass();
$a->GetValue();

What do you expect it to show – ‘Parent’ or ‘Child’? The PHP manual says the result will be ‘Child’. And it seems quite logical to me as I would expect public property to be overridden.

Now rewrite the same code to C#:

class Parent   {
     public string val = "Parent";

     public void GetValue()  {
            Console.WriteLine(this.val);
     }
}

class Child:Parent  {
     public string val = "Child";
}

Child child = new Child();
child.getValue();

Run the code and you will get ‘Parent’…

Change access modifier of val to private in the parent class. Now both PHP and C# outputs the same – ‘Parent’. Why?

First thing to mention is that unlike PHP, public identifiers cannot be overridden in C#. Calling GetValue(), this points to parent object and returns “not overridden” value of val as a result. Private identifers inside parent are not accessible for the child and therefore are not overridden so everything becomes clear.

To make C# behavior similar to PHP you should either add a method with the same signature and with override keyword to Child class and mark parent method as virtual:

class Parent   {
     public string val = "Parent";

     public virtual void GetValue()  {
            Console.WriteLine(this.val);
     }
}

class Child:Parent  {
     public string val = "Child";

     public override void GetValue() {
         Console.WriteLine(this.val);
     }
}

or use overridden public properties to access the val value. This will work correctly as unlike public fields, public properties can be overridden.

class Parent {
    private string privateVal = "Parent";
    public virtual string val
    {
        get { return privateVal; }
    }

    public void GetValue()
    {
        Console.WriteLine(this.val);
    }
}

class Child : Parent {
    private string privateVal = "Child";
    public override string val
    {
        get { return privateVal; }
    }
}

I couln’t find any “standard” behaviour for this case. So I guess it was up to C# and PHP teams how to implement this.