Should one interface inherit another interface

Question

I can\'t seem to find an answer on this and just want to make sure it\'s an ok coding standard. I have interface A that is used by many different classes and don\'

Answer 1

It is certainly possible to have an inheritance tree of interfaces, and even "multiple inheritance" with interfaces. Whether it is the right thing to do or not depends on the interfaces in question. If it really is the case that interface B is an extension or refinement of interface A, then inheritance makes sense, but if the new enum is largely unrelated to the concept expressed by interface A, I would make them two separate interfaces and have the classes that need to implement both interfaces.

Answer 2

Consider whether the interfaces should be logically paired, and if you feel that they work well with each other then absolutely use inheritance.

Lets look at an example;

public interface IScanner
{
    void Scan();
}

public interface IPrinter
{
    void Print();
}

Printers and scanners are often separate objects, each with their own functionality however these two devices are often paired in the same device;

public interface IPhotocopier : IScanner, IPrinter
{
    void Copy();
}

It makes sense that IPhotocopier should inherit from IScanner and IPrinter as this now allows the photocopier to be used as either a scanner or printer (which it contains) in addition to its primary roll as a copier.

Now lets look at one more interface;

public interface IBlender
{
    void Blend();
}

It would not make sense to allow IBlender to be inherited by any of the earlier interfaces (what would you call them? IBlendingScanner?).

If you can't give your new interface a sensible name this might indicate that your may not want to use inheritance in this instance.

It's a bad idea to inherit some interfaces such as IDisposable, since this forces all implementations of your new interface to implement the dispose pattern even if they do not have any disposable resources.

Answer 3

Interface inheritance is an excellent tool, though you should only use it when interface B is truly substitutable for interface A, not just to aggregate loosely-related behaviors.

It's difficult to tell whether it is appropriate for your specific case, but there's nothing wrong using the practice in principle. You see it in the first-rate APIs all the time. To pick just one common example, from the .NET framework:

public interface ICollection<T> : IEnumerable<T>, IEnumerable

Answer 4

IMO this is exactly the right approach, I don't see any problem with it.

Answer 5

I think databases always provide a great way to demonstrate interfaces, so considering if an interface should inherit another interface look at the following,

IMySqlDatabase : IDatabase
MySqlDatabase : IMySqlDatabase

IMsSqlDatabase : IDatabase
MsSqlDatabase : IMsSqlDatabase

A MySqlDatabase IS an IMySqlDatabase and an IMySqlDatabase IS an IDatabase.

Now if you need to make changes to your IDatabase interface it's grandchildren (the conrete database classes) can reap the benefits, but you won't have to expand MySQL AND MsSQL (or perhaps even more DBMS' Interfaces). At the same time in your middle man (IMsSqlDatabase) you can still have interface features that a MySQL or Oracle DB wouldn't support.

Answer 6

Technically speaking, interfaces don't inherit from each other. What really happens when you create an IFoo that inherits from IBar, you're saying that any class that implements IFoo must also implement IBar.

interface IBar
{
    void DoBar();
}

interface IFoo : IBar
{
    void DoFoo();
}

In this example, the IFoo interface does not have a DoBar() method. Most of the time the distinction doesn't matter, but it can bite you when using reflection on an interface rather than a class.