title: Why are Subjects not recommended in .NET Reactive Extensions?
body: <p>I am currently getting to grips with the Reactive Extensions framework for .NET and I am working my way through the various introduction resources I've found (mainly <a href="http://www.introtorx.com">http://www.introtorx.com</a>)</p>
  
  <p>Our application involves a number of hardware interfaces that detect network frames, these will be my IObservables, I then have a variety of components that will consume those frames or perform some manner of transform on the data and produce a new type of frame. There will also be other components that need to display every n'th frame for example.
  I am convinced that Rx is going to be useful for our application, however I am struggling with the implementation details for the IObserver interface.</p>
  
  <p>Most (if not all) of the resources I have been reading have said that I should not implement the IObservable interface myself but use one of the provided functions or classes.
  From my research it appears that creating a <code>Subject&lt;IBaseFrame&gt;</code> would provide me what I need, I would have my single thread that reads data from the hardware interface and then calls the OnNext function of my <code>Subject&lt;IBaseFrame&gt;</code> instance. The different IObserver components would then receive their notifications from that Subject.</p>
  
  <p>My confusion is coming from the advice give in the appendix of <a href="http://www.introtorx.com">this tutorial</a> where it says:</p>
  
  <blockquote>
    <p>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.</p>
  </blockquote>
  
  <p>My application is quite performance critical, I am obviously going to test the performance of using the Rx patterns before it goes in to production code; however I am worried that I am doing something that is against the spirit of the Rx framework by using the Subject class and that a future version of the framework is going to hurt performance.</p>
  
  <p>Is there a better way of doing what I want? The hardware polling thread is going to be running continuously whether there are any observers or not (the HW buffer will back up otherwise), so this is a very hot sequence. I need to then pass the received frames out to multiple observers.</p>
  
  <p>Any advice would be greatly appreciated.</p>