What’s the difference between override and new?

This is all to do with polymorphism. When a virtual method is called on a reference, the actual type of
the object that the reference refers to is used to decide which method implementation to use. When a method
of a base class is overridden in a derived class, the version in the derived class is used, even if the calling
code didn’t “know” that the object was an instance of the derived class. For instance:

public class Base
{
    public virtual void SomeMethod()
    {
    }
}

public class Derived : Base
{
    public override void SomeMethod()
    {
    }
}

...

Base b = new Derived();
b.SomeMethod();

will end up calling Derived.SomeMethod if that overrides Base.SomeMethod. Now,
if you use the new keyword instead of override, the method in the derived class
doesn’t override the method in the base class, it merely hides it. In that case, code like this:

public class Base
{
    public virtual void SomeOtherMethod()
    {
    }
}

public class Derived : Base
{
    public new void SomeOtherMethod()
    {
    }
}

...

Base b = new Derived();
Derived d = new Derived();
b.SomeOtherMethod();
d.SomeOtherMethod();

Will first call Base.SomeOtherMethod (line 3), then Derived.SomeOtherMethod (line 4).
They’re effectively two entirely separate methods which happen to have the same name, rather than the derived method
overriding the base method.

If you don’t specify either new or overrides, the resulting output is the same
as if you specified new, but you’ll also get a compiler warning (as you may not be aware that you’re
hiding a method in the base class method, or indeed you may have wanted to override it, and merely forgot to
include the keyword).

That provides the basics of overriding and the difference between new and override,
but you should really see a book or tutorial for a more in-depth look at polymorphism.

[Author: Jon Skeet]