Blurring the Lines Between Interfaces and Abstract Classes

[Matthew Dennis]

so maybe sealed methods on the interface? They are part of the interface but can't be implemented.

"Time flies like an arrow. Fruit flies like a banana."

[Dean Roddey]

Maybe there's something about C# interfaces I don't get, but in general interfaces are extremely useful. I dunno about this particular aspect of them being discussed here, but in C++ at least they are crucial. Without them, you can't add polymorphic functionality to classes outside of the straight line inheritance mechanism. Do they not work anything like that in C#?

Explorans limites defectum

[Matthew Dennis]

I like interfaces, but I find myself deriving abstract classes from the interface that I derive the implementation classes. I implement the common stuff in the abstract class and leave the rest abstract. This allows me to create multiple base abstract classes for different hierarchies of things that all implement the base interface, but have different common functionality.

"Time flies like an arrow. Fruit flies like a banana."

[David A Gray]

Quote:

I like it. Implement only the methods you really need and not bother about the rest. In Java I have listener interfaces with tons of methods that I will never need but have to put some empty stub code because I have to.

Am I missing something, or are you stuck with some badly designed Java classes? IMO, for the most part, an abstract class should provide a default implementation of every method, perhaps marked as virtual. With that being said, I have one abstract class of my own devising that has one abstract method on it, which must, of course, be implemented by every heir. Since the method takes an enumerated type as its argument, and its work requires evaluating that enumeration by way of a switch block, the base class cannot implement it.

David A. Gray Delivering Solutions for the Ages, One Problem at a Time Interpreting the Fundamental Principle of Tabular Reporting

[Matthew Dennis]

They also allow you to extend the functionality of the entire Interface hierarchy of classes by adding new methods with implementations. Of course, you can already do this with Extension Methods, which allows you to extend things in the context of what your are doing, depending which extension methods you include in your project in your using directives. I can see use cases for both, but I really like Extension Methods for the ability to extend a class without having to do anything to the class itself. Very SOLID.

"Time flies like an arrow. Fruit flies like a banana."

[Lost User]

Well, interfaces are used widely to implement event handlers of various devices. Usually they define methods for many events: public interface FlyOnTheWallListener onConnect, onDisconnect, onReceive, onConnectionClose, onVendorCompanyWentBroke etc ect. They cover every possible event. I need to respond to two events and will have to implement that interface. It's just how it is sometimes.

[Lost User]

I prefer a sealed interface. It ends in much less work :)

It does not solve my Problem, but it answers my question

[Matthew Dennis]

and unsealed wine or scotch

"Time flies like an arrow. Fruit flies like a banana."

[David A Gray]

Quote:

Well, interfaces are used widely to implement event handlers of various devices. Usually they define methods for many events:

I have no objection to interfaces; I use them a lot, and have even defined a few. For instance, my AnyCSV library, available as a NuGet package from [NuGet Gallery: WizardWrx.AnyCSV 7.0.120.30587](https://www.nuget.org/packages/WizardWrx.AnyCSV/), and as an open source project at [GitHub - txwizard/AnyCSV: Parse ANY CSV String, even X.509 Digital Signature Fields!](https://github.com/txwizard/AnyCSV), relies on one that exposes its capabilities to COM.

David A. Gray Delivering Solutions for the Ages, One Problem at a Time Interpreting the Fundamental Principle of Tabular Reporting

[Jorgen Andersson]

Great, just what we need, IGit and ITfs.

Wrong is evil and must be defeated. - Jeff Ello

[Jammer 0]

SO that's one interview question killed ... lol :)

Jammer My Blog | JamSoft

[jsc42]

One of the basic missing features of C# / Java is that you cannot inherit from multiple classes. If you could have multiple inheritance, you could get name clashes where different members of the set of base classes have the same member names or even have classes derived from the same lower level base class (this is known as the diamond problem); either of these scenarios could get name clashes, which is why C# does not support it However, other languages (inc C++?) get over it by explicitly stating which base class's method they are accessing. C# already has mechanisms for resolving name clashes in interfaces. So, converting interfaces into faux abstract classes is a simple way of resolving the diamond problem. It's about time this feature came onboard. I can't be the only one who adds comments to their interfaces giving code for typical implementation of the interface methods. Having the code outside of the comments would greatly simplify implementing interfaces. OK, the new feature is a kludge and a back-door way of creating multiple base class inheritance; but it is simple, effective, and backwards compatible.

[DerekT P]

In the lounge? Really?? :omg:

[Gary Wheeler]

One of my favorite things about C# as opposed to C++ is that they don't get wrapped around the axle over adding useful features that piss off the language dilettantes. This is a useful feature. It lets you implement an interface change piecemeal, rather than forcing you to implement the change in one great steaming pile. You can even have the default implementation perform an assert to help ensure you've caught all cases. The C++ folks would flagellate themselves for years over this, there would be dozens of half-baked implementations with wildly conflicting implementations, and when the standard was finally issued, no one would care.

Software Zen: delete this;

[Nathan Minier]

Sure.

David A. Gray wrote:

1. You cannot directly implement a generic class.

Of course you can. The only constraint is the existence of one or more assigned types, that may or may not be resolved before runtime. I guess if you had a generic abstract class you would need to provide an implementation, but that's because it's abstract, not because it's generic.

David A. Gray wrote:

2. The class provides default implementations of its methods to all instances.

I must be misunderstanding this, because even in terms of fundamental class operations this is false, as it conflates inheritance with instantiation.

"Never attribute to malice that which can be explained by stupidity." - Hanlon's Razor

[Lost User]

C# just tries to catch up with Java 8. Java 8 had this feature since 2014: [https://dzone.com/articles/interface-default-methods-java\](https://dzone.com/articles/interface-default-methods-java)

[Gary Wheeler]

Yes, yes, Jim Gosling is God, Bill Gates is the anti-Christ, and we're all fools for using Micro$haft products. Now that's cleared up, your point?

Software Zen: delete this;

[David A Gray]

Quote:

You cannot directly implement a generic class.

I need to rephrase that; you cannot instantiate an abstract class.

Quote:

2. The class provides default implementations of its methods to all instances

This, too, needs rephrasing; when I create an abstract class, I usually define default implementations of all methods, any of which inheritors are free to override.

David A. Gray Delivering Solutions for the Ages, One Problem at a Time Interpreting the Fundamental Principle of Tabular Reporting

[David A Gray]

Quote:

This is a useful feature. It lets you implement an interface change piecemeal, rather than forcing you to implement the change in one great steaming pile. You can even have the default implementation perform an assert to help ensure you've caught all cases.

Thanks for being the first to show a worthy use case for default implementations of interfaces.

Quote:

he C++ folks would flagellate themselves for years over this, there would be dozens of half-baked implementations with wildly conflicting implementations, and when the standard was finally issued, no one would care.

This I suspect is largely, if not entirely, the result of C# being the responsibility of a single vendor, Microsoft, who has enough muscle to call the shots unequivocally. In contrast, the C++ standard is the work of a committee composed of people from competing vendors, who are motivated by those affiliations to say and do things that favor the people who pay their salaries. The result is compromises and endless delays.

David A. Gray Delivering Solutions for the Ages, One Problem at a Time Interpreting the Fundamental Principle of Tabular Reporting

[David A Gray]

Quote:

t's about time this feature came onboard. I can't be the only one who adds comments to their interfaces giving code for typical implementation of the interface methods. Having the code outside of the comments would greatly simplify implementing interfaces. OK, the new feature is a kludge and a back-door way of creating multiple base class inheritance; but it is simple, effective, and backwards compatible.

Well stated! ;)

David A. Gray Delivering Solutions for the Ages, One Problem at a Time Interpreting the Fundamental Principle of Tabular Reporting