Overriding and calling same method in Base class constructor in C#

Question

My fret is: In the code presented below, it should display A then B. But it displays B then B. Why is it so?

What I feel is, constructor of A gets executed first when creating object of B. In that case, the method in B would not be hitted right? So it should be A.Display() and should result A. Also, then a.Display() should return B because we have override.

So I expect A then B. Because its not overloading but overriding. I know the definition of these things, I expect to understand the reason for this behavior and how it works internally as I am not convinced with BB but AB.

---

Code

class A
{
    public A()
    {
        this.Display();
    }
    public virtual void Display()
    {
        Console.WriteLine("A");
    }
}

class B :A
{
    public override void Display()
    {
        Console.WriteLine("B");
    }
}

class C
{
    static void Main()
    {
        A a = new B();
        a.Display();
        Console.WriteLine();
        Console.ReadLine();
    }
}

---

Outputs

1) On override of Display method in the derived class yields the following:

A a = new A(); // ---> AA
B a = new B(); // ---> BB // I expect AB.
A a = new B(); // ---> BB // I expect AB.

2) using the NEW keyword in the Display method in derived class yields the following:

B a = new B(); // ---> AB // I Expect AA here.
A a = new B(); // ---> AA
A a = new A(); // ---> AA

3) More interesting findings are:

When I use base.Display() in the derived constructor with override of the base method in derived class, it gives me BAB

I do not see any logic at least in this. because, it should give BBB

Answer 1

What I feel is, constructor of A gets executed first when creating object of B.

Correct.

In that case, the method in B would not be hit right?

This is incorrect.

In similar code in C++ you would be correct. In C++ there is a rule that virtual function dispatch tables are built as the object is being constructed. That is, when the "A" constructor is entered, the vtable is filled in with the methods in "A". When control goes to the "B" ctor, the vtable is then filled in with the methods of B.

This is not the case in C#. In C# the vtable is filled in the moment the object comes out of the memory allocator, before either ctor is executed, and it does not change after that. The vtable slot for the method always contains the most derived method.

Therefore calling a virtual method in a ctor as you are doing here is a very bad idea. A virtual method can be called where the implementation is on a class whose ctor has not run yet! It might therefore depend on state that has not yet been initialized.

Note that field initializers run before all ctor bodies, so fortunately an override on a more derived class will always run after the field initializers of the overriding class.

The moral of the story is: simply don't do that. Don't ever call a virtual method in a ctor. In C++ you might get a different method than you expect, and in C# you might get a method that uses state that is not initialized. Avoid, avoid, avoid.

Why we shouldn't call virtual methods inside ctor? Is it because we get only the (latest derived) results only) always in the vtable?

Yes. Let me illustrate with an example:

class Bravo
{
    public virtual void M() 
    {
        Console.WriteLine("Bravo!");
    }
    public Bravo()
    {
        M(); // Dangerous!
    }
}
class Delta : Bravo:
{
    DateTime creation;
    public override void M() 
    {
        Console.WriteLine(creation);
    }
    public Delta() 
    {
        creation = DateTime.Now;
    }
}

OK, so the expected behavior of this program is that when M is called on any Delta, it will print out the time that the instance was created. But the order of events on new Delta() is:

• Bravo ctor runs
• Bravo ctor calls this.M
• M is virtual and this is of runtime type Delta so Delta.M runs
• Delta.M prints out the uninitialized field, which is set to the default time, not the current time.
• M returns
• Bravo ctor returns
• Delta ctor sets the field

Now do you see what I mean when I say that the overriding method might rely on state that is not initialized yet? In any other usage of M, this would be fine because the Delta ctor would already be finished. But here M is called before the Delta ctor even starts!

Answer 2

You create an instance of an object B. It uses the code of the constructor that is defined on class A as you did not override it in B. But the instance is still B, so other methods called in the constructor are the ones defined in B, not A. Hence you see the result of Display() defined in class B.

Update based on update of the question

I'll try to explain the "weird" results you're getting.

When overriding:

B a = new B(); // ---> BB // I expect AB.

A a = new B(); // ---> BB // I expect AB.

This is covered above. When you override a method on a child class, this method is used if you're using an instance of the child class. This is a basic rule that the methods used are decided by the class of the instance of a variable, not by the class used to declare the variable.

When using new modifier for the method (hiding inherited method)

B a = new B(); // ---> AB // I Expect AA here.

Now there's two different behaviours here:

• When the constructor is used, it's using the constructor in class A. As the inherited method is hidden in the child class, the constructor is using Display() method from class A and hence you see A printed.

• When you later call Display() directly, the instance of the variable is B. For that reason, it uses the method defined on class B which prints B.