do superclass and subclass objects share the common method?

Question

If I don't mark a method as virtual, will it be available to a derived class?

If using object a, I make changes to method of base class. Then object b accesses the same method m1() of the base class (which is available to derived class).

Will it print those changed value by object a?

Will they share common method ?

class A
{
    public int m(int i)
    {
        return i * i;
    }
}

class B : A
{
}

class C
{
    static void Main()
    {
        A a = new A();
        int x = a.m(2);    // returns 4

        B b = new B();
        int y = b.m(4);    // 16
    }
}

Answer 1

if i don't use virtual to a method, will it be available to derived class?

Yes the method will be available to any derived classes but you won't be able to override this method in a derived class and change its behavior. In your example class B posses the m1 method because it derives from class A but cannot override it as the method is not virtual.

As a matter of fact your example won't even compile because you didn't specify a return type for the m1 method. Also by not specifying an access modifier for the method, private will be assumed and you won't even be able to call the base method in derived classes (unless using reflection of course). (after edit by @dtb the code will now compile)

Answer 2

Yes, because the derived class is a type of the base class.

Consider a Mammal class. All mammals breathe, so we would have Mammal.Breathe(). Now consider a Cat class. Since cats are mammals, we would have this as derived from Mammal, then there is already a Cat.Breathe() inherited from Mammal, without any extra work (the "no extra work" bit being one time-saving aspect of OO).

If Mammal.Breathe() was virtual, then we could make it behave differently in the case of Cat.Breathe(). If it's not virtual, we cannot, though a non-virtual method can call a virtual method, which would make that part of its behaviour overridable.