new vs override keywords

问题

I've got a question concerning polymorphic methods. I've got two classes: the base class with the non-virtual method Foo( ) which calls its virtual method Foo (int i) (like this: Foo() {Foo(1);}) and the derived class which overrides method Foo(int i).

If I call Foo() method of an instance of the derived class the walkthrough is as the following: base Foo() -> override Foo(int i). But if I change override method to new the walkthrough is as the following: base Foo -> base Foo(int i). It doesn't even get to the new Foo(int i) method. Please, explain the sequence of those methods and why it is the way it is.

using System;
class Program
{
    sealed void Main()
    {
        DerivedClass d = new DerivedClass();
        //Goes to BaseClass Foo() method
        //then goes to Derived Foo(int i ) method
        d.Foo();
    }
}
class BaseClass
{
    public void Foo() { Foo(1); }
    public virtual void Foo(int i) { // do something;
    }
}
class DerivedClass : BaseClass
{
    public override void Foo(int i)  { //Do something
    }
}

//////////////////////////////////////////////////////////////////////

using System;
    class Program
    {
        sealed void Main()
        {
            DerivedClass d = new DerivedClass();
            //Goes to BaseClass Foo() method
            //then goes to base Foo(int i) method
            //never gets to Foo(int i)  of the derived class
            d.Foo();
        }
    }
    class BaseClass
    {
        public void Foo() { Foo(1); }
        public virtual void Foo(int i) { // do something;
        }
    }
    class DerivedClass : BaseClass
    {
        public new void Foo(int i)  { //Do something
        }
    }

回答1:

(When using new.)

It doesn't even get to the new Foo(int i) method.

Yes it does - but it executes the BaseClass implementation of Foo(int) because it's not overridden in the derived class. That's the whole point of new - it's saying, "I'm not overriding a base class method - I'm a whole new method." If you want to override the base class method, use override. The clue is in the keyword :)

So for example, when using new:

BaseClass x = new DerivedClass();
x.Foo(1); // Calls BaseClass.Foo(int)

DerivedClass y = new DerivedClass();
y.Foo(1); // Calls DerivedClass.Foo(int)

But when using override:

BaseClass x = new DerivedClass();
x.Foo(1); // Calls DerivedClass.Foo(int) // Due to overriding

DerivedClass y = new DerivedClass();
y.Foo(1); // Calls DerivedClass.Foo(int)

回答2:

You may want to take a look at this: http://msdn.microsoft.com/en-us/library/ms173153.aspx

Which starts as follows:

In C#, a method in a derived class can have the same name as a method in the base class. You can specify how the methods interact by using the new and override keywords. The override modifier extends the base class method, and the new modifier hides it. The difference is illustrated in the examples in this topic.

This behavior is by design.

来源：https://stackoverflow.com/questions/20577383/new-vs-override-keywords

标签

polymorphism

overriding