Question

MSDN says:

When used as a modifier, the new
keyword explicitly hides a member
inherited from a base class. When you
hide an inherited member, the derived
version of the member replaces the
base-class version. Although you can
hide members without the use of the
new modifier, the result is a warning.
If you use new to explicitly hide a
member, it suppresses this warning and
documents the fact that the derived
version is intended as a replacement.

Example:

class Base
{
 int value;

 virtual bool Foo()
 {
   value++;
 }
}

class Derived : Base
{
 int value;

 override bool Foo()
 {
  value++;
 }

}

Do I have to add new modifier to Derived.value declaration? What changes?

Answer 1

Since the value field is private, it’s not accessible in the derived class. Thus, the declaration in the derived class does not really hide anything. You shouldn’t add new to the declaration. If you do, nothing changes, except the compiler will warn you about using new incorrectly. If the value field was accessible in the derived class (e.g. it was public), then you should have used new to express your intention to hide the base member:

class A {
    public int field;
}
class B : A {
    public int field; // warning. `B.field` hides `A.field`. 
}

Using new will silence that warning (it will have no other effect):

class B : A {
    public new int field; // Dear compiler, shut up please.
}

You can’t declare a method as both override and new. They are mutually exclusive.