Liskov substitution principle and Streams

Question

Does the fact that there are Stream derived classes that cannot be written or sought break the Liskov substitution principle?

For example, the NetworkStream cannot b

Answer 1

The Can... methods mean that Stream doesn't break LSP. Stream provides the capability to read, write and seek, but no guarantee that any implementing class will honour it. The Can... methods make this an explicit feature of the Stream contract - derived classes must implement them to allow client code to check whether a derived class implements certain behaviour before a call is made. So, any code that attempts to write to a Stream should check CanWrite before calling Write, for example, and this can be done with any correctly-implemented derivative of Stream. Ergo, they're interchangeable, as LSP requires.

I think it's certainly true that adding methods to flag whether or not derived classes implement a particular feature could be abused - if a team were undisciplined, they may end up with a very broad, bloated interface that breaks ISP. I think that Stream and IList are well-designed in this respect - they don't break LSP, and they define a narrow enough contract of closely-related behaviours to stay within ISP. Clearly, their design has been considered.

I think that in the case of Square inheriting from Rectangle, you could certainly add DoesHeightAffectsWidth and DoesWidthAffectsHeight to fix the issue, but the team must decide whether that's acceptable, or whether the addition of those methods breaks ISP. Is the addition of AreAllInternalAnglesEqual to support trapezoids too far? To a certain extent, it's up to the engineers writing the code.