Are questions about interfaces/protocols and APIs on-topic?

Yes. But that does not mean that the question itself is on-topic. There are still other factors to consider, such as: Is there enough context in the question? Is it clear how these interfaces will be used?

I see no reason whatsoever to create a rule that says "Interface-only questions are off-topic. Period." Each question needs to be looked at individually.

For example, have Code Review suffered from having considered this question as on-topic? Or this question? I don't think so. Likewise, if more questions with high-quality like this appear, I see no reason that they should be closed for being off-topic.

Are questions about interfaces/protocols and APIs on-topic?

Sometimes.

Are questions about interfaces/protocols and APIs off-topic?

Sometimes.

I don't think we need a rule stating that interface questions are on-topic or off-topic. It all depends on the rest of the question.

I'm stating that questions about interfaces can be on-topic.

What does it take for a question about interfaces to be on-topic?

• Good description of the purpose of the interface
• Sufficient amount of code to answer the question
• Clear question

If you consider an interface-only question off-topic, then it is more likely because it is unclear what you are asking.