In general, it is important to be courteous and respectful while also being very clear and helpful to the developer whose code you are reviewing. One way to do this is to be sure that you are always making comments about the code and never making comments about the developer. You don’t always have to follow this practice, but you should definitely use it when saying something that might otherwise be upsetting or contentious. For example:
Bad: “Why did you use threads here when there’s obviously no benefit to be gained from concurrency?”
Good: “The concurrency model here is adding complexity to the system without any actual performance benefit that I can see. Because there’s no performance benefit, it’s best for this code to be single-threaded instead of using multiple threads.”
One thing you’ll notice about the “good” example from above is that it helps the developer understand why you are making your comment. You don’t always need to include this information in your review comments, but sometimes it’s appropriate to give a bit more explanation around your intent, the best practice you’re following, or how your suggestion improves code health.
In general it is the developer’s responsibility to fix a CL, not the reviewer’s. You are not required to do detailed design of a solution or write code for the developer.
This doesn’t mean the reviewer should be unhelpful, though. In general you should strike an appropriate balance between pointing out problems and providing direct guidance. Pointing out problems and letting the developer make a decision often helps the developer learn, and makes it easier to do code reviews. It also can result in a better solution, because the developer is closer to the code than the reviewer is.
However, sometimes direct instructions, suggestions, or even code are more helpful. The primary goal of code review is to get the best CL possible. A secondary goal is improving the skills of developers so that they require less and less review over time.
Remember that people learn from reinforcement of what they are doing well and not just what they could do better. If you see things you like in the CL, comment on those too! Examples: developer cleaned up a messy algorithm, added exemplary test coverage, or you as the reviewer learned something from the CL. Just as with all comments, include why you liked something, further encouraging the developer to continue good practices.
If you ask a developer to explain a piece of code that you don’t understand, that should usually result in them rewriting the code more clearly. Occasionally, adding a comment in the code is also an appropriate response, as long as it’s not just explaining overly complex code.
Explanations written only in the code review tool are not helpful to future code readers. They are acceptable only in a few circumstances, such as when you are reviewing an area you are not very familiar with and the developer explains something that normal readers of the code would have already known.