This article moves beyond the typical “mechanical” distinctions between interfaces and abstract classes, focusing instead on their intended roles in your code.
In C#, an interface establishes a contract that implementing classes must follow. When multiple implementations share common functionality, they may encapsulate that shared behavior in an abstract base class. However, not every subclass of an abstract base class is required to implement the associated interface. The decision to implement an interface always rests with the developer writing the class.
It’s also essential to recognize that implementers of an interface won’t always inherit from the same base class—nor should they. Inheritance should only be used when a group of implementations naturally forms a cohesive “family” of related specializations, and when structuring them in a class hierarchy provides clear benefits. Regardless of how these implementations are structured internally, clients should always interact with the interface, keeping the underlying class hierarchy abstracted away.
Even if an abstract base class implements an interface, this should be considered a convenience, not a design constraint. Clients should never rely directly on abstract classes, as they represent implementation details that may evolve over time. The core purpose of an interface is to define and enforce the client’s requirements while giving implementers the flexibility to determine how best to fulfill them—without rigid dependencies on internal code-sharing mechanisms.
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