reactive/Rx.NET/Documentation/IntroToRx/18_UsageGuidelines.md

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title : Appendix A Usage guidelines

#Appendix {#Appendix .SectionHeader}

#Usage guidelines {#UsageGuidelines}

This is a list of quick guidelines intended to help you when writing Rx queries.

• Members that return a sequence should never return null. This applies to IEnumerable<T> and IObservable<T> sequences. Return an empty sequence instead.
• Dispose of subscriptions.
• Subscriptions should match their scope.
• Always provide an OnError handler.
• Avoid breaking the monad with blocking operators such as First, FirstOrDefault, Last, LastOrDefault, Single, SingleOrDefault and ForEach.
• Avoid switching between monads, i.e. going from IObservable<T> to IEnumerable<T> and back to IObservable<T>.
• Favor lazy evaluation over eager evaluation.
• Break large queries up into parts. Key indicators of a large query:
  
  1. nesting
  2. over 10 lines of query comprehension syntax
  3. using the into statement
• Name your queries well, i.e. avoid using the names like query, q, xs, ys, subject etc.
• Avoid creating side effects. If you must create side effects, be explicit by using the Do operator.
• Avoid the use of the subject types. Rx is effectively a functional programming paradigm. Using subjects means we are now managing state, which is potentially mutating. Dealing with both mutating state and asynchronous programming at the same time is very hard to get right. Furthermore, many of the operators (extension methods) have been carefully written to ensure that correct and consistent lifetime of subscriptions and sequences is maintained; when you introduce subjects, you can break this. Future releases may also see significant performance degradation if you explicitly use subjects.
• Avoid creating your own implementations of the IObservable<T> interface. Favor using the Observable.Create factory method overloads instead.
• Avoid creating your own implementations of the IObserver<T> interface. Favor using the Subscribe extension method overloads instead.
• The subscriber should define the concurrency model. The SubscribeOn and ObserveOn operators should only ever precede a Subscribe method.

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