Timeline for Why was C# made with "new" and "virtual+override" keywords unlike Java?

Current License: CC BY-SA 3.0

14 events

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Dec 11, 2018 at 2:15	comment	added	Soroush Falahati		Also, this should be added that `new` doesn't necessarily hide the method or property of the base class. It works only when you do know the real type of object. If a part of code accepts, for example, the base class as a parameter or the value, it stills calls the base class's method, even tho it is hidden in the child class. `new` use cases are little and far between, however, when you need them, they are super useful.
Dec 11, 2018 at 2:11	comment	added	Soroush Falahati		Another good use case for `new` is changing the signature of a property or method. A most useful example that I used multiple times was to change the type of a property to a subclass of the base class's property. For example hiding `IXXResolver Resolver {get;}` with `new DefaultResolver Resolver {get => base.Resolver as DefaultResolver; }`. It fits perfectly. It does still respect the contract with the base class and also makes it easier to access type-specific properties of a subclass's type (or interface implementation) if you already know the type of the child class.
Jun 20, 2014 at 10:09	comment	added	deworde		@svick Correct, which is why it's normally a compiler warning. But having the ability to "new" covers edge conditions (such as the one given), and even better, makes it really obvious that you're doing something weird when you come to diagnose the inevitable bug. "Why's this class and only this class buggy... ah-hah, a "new", let's go test where the SuperClass is used".
Jun 18, 2014 at 18:28	comment	added	supercat		@svick: What would you think of defining a `new` function but tagging it `Obsolete("Use XX or YY depending upon requirements", true)` and refactoring code which expects the derived-class meaning to use a slightly-renamed alternative? That should avoid hidden bugs.
Jun 18, 2014 at 17:18	comment	added	pdr		@C.Champagne: Any tool can be used badly and this one is a particularly sharp tool -- you can cut yourself easily. But that's not a reason to remove the tool from the toolbox and remove an option from a more talented API designer.
Jun 18, 2014 at 17:08	comment	added	C.Champagne		@pdr I don't know C# very well but I think understood the difference and that is why I used "almost". That is for me the biggest difference between those languages. The thing is that it breaks polymorphism, that is why I don't feel enthousiastic.
Jun 18, 2014 at 17:02	comment	added	Bobson		@C.Champagne - It's the difference between "All child classes should behave like the base class specifies" vs "All child classes can do what they want". One gives you much more control.
Jun 18, 2014 at 16:55	comment	added	pdr		@C.Champagne: You're still confusing `override` with `new`. They're not the same; not even close.
Jun 18, 2014 at 16:47	comment	added	C.Champagne		Maybe am I wrong but you can do (almost) all of this with Java. The thing is that the philosophy is the opposite. In Java, everything is overridable by default but you can blacklist the methods you don't want to override with the `final`keyword.
Jun 18, 2014 at 16:46	comment	added	pdr		One excellent use of `new` is in WebViewPage<TModel> in the MVC framework. But I have also been thrown by a bug involving `new` for hours, so I don't think it's an unreasonable question.
Jun 18, 2014 at 16:01	comment	added	Bobson		@svick - Potentially, yes. In our particular scenario, that would never occur, but that's why I added the caveat.
Jun 18, 2014 at 15:57	comment	added	svick		I think that intentionally using `new` this way is a bug waiting to happen. If I call a method on some instance, I expect it to always do the same thing, even if I cast the instance to its base class. But that's not how `new`ed methods behave.
Jun 18, 2014 at 15:51	history	edited	Bobson	CC BY-SA 3.0	Adding example.
Jun 18, 2014 at 15:42	history	answered	Bobson	CC BY-SA 3.0