Generally, the rule goes something like this:
- Inheritance describes an is-a relationship.
- Implementing an interface describes a can-do relationship.
To put this in somewhat more concrete terms, let’s look at an example. The System.Drawing.Bitmap class is-an image (and as such, it inherits from the Image class), but it also can-do disposing, so it implements the IDisposable interface. It also can-do serialization, so it implements from the ISerializable interface.
But more practically, interfaces are often used to simulate multiple inheritance in C#. If your Processor class needs to inherit from something like System.ComponentModel.Component, then you have little choice but to implement an IProcessor interface.
The fact is that both interfaces and abstract base class provide a contract specifying what a particular class can do. It’s a common myth that interfaces are necessary to declare this contract, but that’s not correct. The biggest advantage to my mind is that abstract base classes allow you provide default functionality for the subclasses. But if there is no default functionality that makes sense, there’s nothing keeping you from marking the method itself as abstract, requiring that derived classes implement it themselves, just like if they were to implement an interface.
For answers to questions like this, I often turn to the .NET Framework Design Guidelines, which have this to say about choosing between classes and interfaces:
In general, classes are the preferred construct for exposing abstractions.
The main drawback of interfaces is that they are much less flexible than classes when it comes to allowing for the evolution of APIs. Once you ship an interface, the set of its members is fixed forever. Any additions to the interface would break existing types implementing the interface.
A class offers much more flexibility. You can add members to classes that you have already shipped. As long as the method is not abstract (i.e., as long as you provide a default implementation of the method), any existing derived classes continue to function unchanged.
[ . . . ]
One of the most common arguments in favor of interfaces is that they allow separating contract from the implementation. However, the argument incorrectly assumes that you cannot separate contracts from implementation using classes. Abstract classes residing in a separate assembly from their concrete implementations are a great way to achieve such separation.
Their general recommendations are as follows:
- Do favor defining classes over interfaces.
- Do use abstract classes instead of interfaces to decouple the contract from implementations. Abstract classes, if defined correctly, allow for the same degree of decoupling between contract and implementation.
- Do define an interface if you need to provide a polymorphic hierarchy of value types.
- Consider defining interfaces to achieve a similar effect to that of multiple inheritance.
Chris Anderson expresses particular agreement with this last tenet, arguing that:
Abstract types do version much better, and allow for future extensibility, but they also burn your one and only base type. Interfaces are appropriate when you are really defining a contract between two objects that is invariant over time. Abstract base types are better for defining a common base for a family of types.