Problems in multiple interface inheritance

Question

I have a reasonably large application that has a lot of base classes. I want to change this into interfaces, to allow (easier) unit testing.

However, when I make a new 'design' of the interfaces I might use, I notice that some classes will inherit from 5 or more interfaces. Can this cause problems?

I ask this since I don't want to refactor the whole application to find out it will not work; or if there is a nice solution, to take this into the refactoring steps.

Some of the interfaces I had in mind (pseudo code). It is incomplete but it shows the interface relations I want to make (there will be many more). E.g. IBank derives from 5 interfaces, and I think more will be needed.

INavigation: IOffset
   INavigation Root { get; }
   INavigation Parent { get; }

IMemory
   string FileName { get; set; }
   byte[] Content { get; }
   Model Model { get; }

IPcgMemory: IMemory
   ProgramBanks ProgramBanks { get; }
   CombiBanks CombiBanks { get; }
   SetLists SetLists { get; }
   DrumKitBanks { get; }
   WaveSequenceBanks WaveSequenceBanks { get; }


ISelectable
   bool IsSelected { get; set; }

ICountable
   int Count { get; }

IName
   string Name { get; set; }

IOffset
   int ByteOffset { get; set; }


IPatch: IName, ISelectable, IOffset, INavigation


IBank: ICountable, IName, ISelectable, IOffset, ICountable, INavigation
   IEnumerable<Patch> Patches { get; }

IProgramBank: IBank

ICombiBank: IBank

ISetList: IBank

IWaveSequenceBank: IBank

IDrumKitBank: IBank


IBanks: ICountable, INavigation, ISelectable, IOffset, ICountable
   IEnumerable<IBank> Banks { get; }

IProgramBanks : IBanks
   int CountModeledPrograms { get; }
   int CountSampledPrograms { get; }

ICombiBanks: IBanks

ISetLists: IBanks

IWaveSequenceBanks: IBanks

IDrumKitBanks: IBanks

PcgMemory: IPcgMemory

ProgramBanks: IPatchCollection

Sometimes we tend to "over engineer". You might want to try to find a common logical group for a couple of these interfaces so you can join them together. — Yuval Itzchakov
– Yuval Itzchakov, Commented Dec 12, 2014 at 9:33

Patrick Hofman · Accepted Answer · 2014-12-12 09:33:58Z

4

I wonder what problems you expect. List<T> derives from 8 interfaces (IList<T>, ICollection<T>, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable) and I have never seen the problem with it.

The problem might be when you are going to test this, having that much relationships between interfaces, it takes a lot of work to create test objects, but that comes with the job I think. Try to find out if you really need all those interfaces, maybe you can combine them or skip them.

answered Dec 12, 2014 at 9:33

Patrick Hofman

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6 Comments

Yuval Itzchakov Over a year ago

A List<T> is something you implement once. If any of the types he creates in his project has to implement all these multiple interfaces, it can get messy.

Patrick Hofman Over a year ago

@YuvalItzchakov: True, but the initial question is: I notice that some classes will inherit from 5 or more interfaces. Can this cause problems? and that answers to know. The second paragraph points on the messy stuff.

Patrick Hofman Over a year ago

@YuvalItzchakov: And do you know how many classes implement IList? A lot.

Michel Keijzers Over a year ago

Thanks for this answer, it seems that I can best start making the interfaces as small as possible and only extend if really needed.

Patrick Hofman Over a year ago

@MichelKeijzers: Sometimes it is just a matter of one addition property which could be added to an existing interface, or creating a new one. Try to keep it simple.

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