system.reactive - RX Subjects - are they to be avoided?

Question

I've had a mini-discussion on the topic in another thread, and would like to have people's input on the "bad" sides of subjects.

People who frequent the RX forum know that E.Meijer does not like Subjects. While I have a deepest respect to RX creator's opinion, I have been using Subjects quite extensively in multiple projects for a couple of years and haven't had any architectural problem or a bug because of them.

The only "pitfall" with Subjects I can name is that they are not "reusable" - after you have completed an observable on a Subject, you need to re-instantiate it, before new subscribers can receive events from it.

"Code smell" and "Don't like them" need to be supported by "pragmatic" examples - can you bring to our attention possible situations when using a Subject can lead to a bug or a problem? Or maybe you think they are easy and harmless altogether - then try to define an area where they are to be used.

Answer 1

Erik Meijer is thinking in a purely functional way - Subjects are the mutable variables of Rx. So, in general usage he's right - using Subjects is sometimes a way to cop out of Thinking Functionally, and if you use them too much, you're trying to row upstream.

However! Subject are extremely useful when you're interfacing with the non-Functional world of .NET. Wrapping an event or callback method? Subjects are great for that. Trying to put an Rx "interface" onto some existing code? Use a Subject!