CODEWITHSUNDEEP
c#constructorvirtual-functions
Buzz
Buzz

Reputation: 6330

Why override method of SecondChild class is not getting invoked twice?

I am not clear about why the SecondChild class DoSomething is not getting called again when Child class is getting initialized.

class Parent
{
    public Parent()
    {
       DoSomething();
    }
    protected virtual  void DoSomething()
    {
        Console.WriteLine("Parent Method");
    }
}

class Child : Parent
{
    private string foo;

    public Child()
    {
        foo = "HELLO";
    }
    protected override void DoSomething()
    {
       Console.WriteLine(foo.ToLower());
    }
}

class SecondChild : Parent
{
    public SecondChild()
    {
        var c = new Child();
    }

    protected override void DoSomething()
    {
        Console.WriteLine("In second Child");
    }
}

class Program
{
    static void Main(string[] args)
    {
        SecondChild c = new SecondChild();
        Console.ReadLine();
    }
}

I was expecting that DoSomething() of SecondChild will be called twice here, but instead Child class DoSomething() is invoked which will give NullException.

Upvotes: 4

Views: 814

Answers (2)

Tarek Abo ELkheir
Tarek Abo ELkheir

Reputation: 1331

Well DoSomething() its get called when create an instance of SecondChild class, but when you create and instance of class Child, first it execute the constructor of Parent class, which is calling DoSomething method of the child class which is OK, but because the constructor of Child class is not executed yet, foo field is not yet and doing foo.ToLower() throws null reference exception.

DoSomething is getting called twice once of Child class and once for SecondChild, but for Child class is throwing exception because of foo is null

So the tricky part here the base constructor get executed before the constructor of the derived class

Upvotes: 0

Tatranskymedved
Tatranskymedved

Reputation: 4381

I have adjusted Your definition a bit:

class Parent
{
    protected string foo;
    public Parent()
    {
        foo = "Parent1";
        DoSomething();
        foo = "Parent2";
    }
    protected virtual void DoSomething()
    {
        Console.WriteLine("Parent Method");
    }
}

class Child : Parent
{

    public Child()
    {
        foo = "HELLO";
    }
    protected override void DoSomething()
    {
        Console.WriteLine(foo.ToLower());
    }
}

class SecondChild : Parent
{
    public SecondChild()
    {
        var c = new Child();
    }

    protected override void DoSomething()
    {
        Console.WriteLine("In second Child");
    }
}

class Program
{
    static void Main(string[] args)
    {
        SecondChild c = new SecondChild();
        Console.ReadLine();
    }
}

Output for this will be:

In second Child

parent1

Reason why? Look at the method call order:

new SecondChild()
  -> SecondChild:base()
      -> base.DoSomething() //virtual
    -> SecondChild.DoSomething()
  -> new Child()
    -> Child:base()
      -> base.DoSomething() //virtual
    -> Child.DoSomething()

Upvotes: 4

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