In C#, when should you use interfaces and when should you use abstract classes? What can be the deciding factor?
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The advantages of an abstract class are:
Interfaces are merely data passing contracts and do not have these features. However, they are typically more flexible as a type can only be derived from one class, but can implement any number of interfaces.
The real question is: whether to use interfaces or base classes. This has been covered before.
In C#, an abstract class (one marked with the keyword "abstract") is simply a class from which you cannot instantiate objects. This serves a different purpose than simply making the distinction between base classes and interfaces.
Abstract classes and interfaces are semantically different, although their usage can overlap.
An abstract class is generally used as a building basis for similar classes. Implementation that is common for the classes can be in the abstract class.
An interface is generally used to specify an ability for classes, where the classes doesn't have to be very similar.
Another thing to consider is that, since there is no multiple inheritance, if you want a class to be able to implement/inherit from your interface/abstract class, but inherit from another base class, use an interface.
Source: Stackoverflow.com