Let me preface this with 'I am no expert'.

In the last app I wrote, I implemented that functionality in classes, because I knew I was always going to write to the same data store and the code was very generic for that datastore.

If I thought I would ever have to write to different or multiple datastores, I think I would implement that functionality via interfaces, because the implementation is likely to be different for each one.

>So if my objects are loaded and saved to a database, should I have them use an interface with Save(), Load(), Delete(), etc? Maybe I get confused because I am maintaining code that ineffectively uses them.
>
>>Here is the best explanation I've read...
>>
>>"...think at a much higher level. When you're designing an object model:
>>
>>* interfaces describe "can do" relationships.
>>* abstract classes describe "is a" relationships.
>>
>>Or, to put it another way
>>
>>* interfaces share appearance but not implementation
>>* abstract classes share both appearance and implementation
>>
>>When you're trying to decide whether to use an interface or an abstract class, ask yourself:
>>
>>* is there any situation where two COMPLETELY different objects would need to be treated as the same?
>>
>>* is there a fundamental "core" of implementation that needs to be shared by EVERY implementation?
>>
>>* is there a sensible "is a" relationship? A Giraffe IS A Mammal IS AN animal.
>>
>>* is there a sensible "can do" relationship? A Bitmap CAN DO disposal. "
>>
>>Eric Lippert
>>
>>from http://discuss.techinterview.org/default.asp?joel.3.89283.9
>>
>>Our application allows for customization on a per-client basis. We define a class that implements an interface for the customization. The class is instanciated using a factory method. Because it implements an interface, we know what methods are available and can call them appropriately, however, the implementation is different for each customer. I tried to implement this with an abstract class but had trouble getting it to work. No problems with an interface.
>>
>>
>>>I know there was a thread a few weeks ago, but I can't find it.
>>>
>>>I have been reading about interfaces, and I understand the concept, but my question is "So What?" I just don't see them being helpful other than to enforce standards. Should all of my classes have an interface? What does that gain me?
>>>
>>>Also, when should I use an interface as opposed to inheritance? I am just not "getting" it and I figured it was about time to do something about it.