what is “public new virtual void Method()” mean?

Question

when use new virtual key words to decorate the method? what is the affection? Like define an interface, and add a class to inherit the interface. but use the new virtual to

Answer 1

The new keyword used like this is member hiding.

I have never seen it used in conjunction with the virtual keyword, mind you. It is simply allowing types that derive from PrinterTwo to override the Print method implementation.

The new keyword used this way allows a type to hide the members of base types, but only if you are using a variable of the type itself.

For example, if you were to do:

PrinterOne one = new PrinterTwo();
one.Print();

It would not call the method in PrinterTwo as it is not part of the inheritance chain.

As for when you would do this... when you really, really need to for some odd reason that I can't think of (reflection maybe?) and you cannot edit the code in PrinterOne.

Personally, I wouldn't ever do this.

As for why the output is printer one... calling IPrinter.Print will call against the type it is defined on (PrinterOne in this case), which will put you back in my above example about the new keyword being ignored unless you talk to the type that features it.

Basically, using IPrinter is analogous to using PrinterOne in my small example above.

To solve the problem, make the PrinterOne method virtual and completely remove the use of new virtual in PrinterTwo.