Reactive Extensions to .Net; a quick-start

This aims to get to the point with Reactive Extensions to .Net. If you want to drill down further, I suggest you read the fanstastic book at IntroToRx.

If you get bored, at least read the summary and advice at the end.


IObservable<T> is a stream of T which an IObserver<T> can be subscribed to, a-la observable.Subscribe(observer).

Thus, an IObserver<T> observes instances of T.

An IObservable<T> will call .OnNext(), .OnError() or .OnCompleted() methods on each observer. When an observer receives .OnComplete() or .OnError(), it should expect no further errors or values.

There are overloads / extension methods for IObservable<T>.Subscribe which allow you to subscribe with a (T t) => {}, rather than an IObserver<T> implementation.


Subject<T> implements both interfaces. Anything pushed to it via IObserver<T>.OnNext is pushed to it’s own observers. Observers are registered through the subject’s implementation of the IObservable<T> interface.

Subject<T> implements ISubject<T>, which itself implements ISubject<TSource, TResult> where both types are the same. That is; it is entirely possible for a subject to accept one type but publish another.

????? Subject.Create() accepts an IObserver<TSource> and an IObservable<TResult>. This is the easiest way to utilise a subject with different input and output types.

ReplaySubject<T> caches values; observers will receive events even if they subscribe after events are published. A replay subject has a configurable bufferSize and a configurable time window.

BehaviourSubject<T> always contains a buffer of one T - so something which observes it will immediately get one value. When you construct a BehaviourSubject<T> you must provide the intitial T.

AsyncSubject<T> buffers one value but doesn’t publish it until it’s .OnComplete() method is called.

Lifetime Management

Subscribe() will return an IDisposable. You should dispose it when you want to cancel the subscription. Consider utilizing C#’s using (){} statement.

There are overloads which accept cancellation tokens. Be careful not to end up with subscriptions which are never cancelled.

Creating sequences

Observable has static methods which return IObservable<T> implementations:

Observable.Return<string>("Value") // Returns one value to observer
Observable.Empty<string>() // Calls OnComplete() instantly
Observable.Never<string>() // Never completes
Observable.Throw<string>(e) // Calls OnError(e) instantly

An Observable.Create() method takes a factory delegate. The factory delegate is called every time an observer subscribes to the create observable. The expected return type is the IDisposable which should be given back to the call site (see Lifetime Management).

Observable.Generate() exists for creating sequences. It accepts a start value and three delegates: .Generate(startValue, shouldContinue, generateNextValue, getCurrentValue).

Observable.Interval() and Observable.Timer() call their observers over time.

Observable.Start(fn) calls fn repeatedly, publishing whatever it returns.

You get the idea. Check out these other ways to create observables:


Reduce, Join, Aggregate, Map

Rx provides LINQ-esque methods over observables.

You’re able to .Where etc and .Subscribe to that result. .Select, .SelectMany, etc shouldn’t be a surprise to you - they turn an IObservable<T1> in to an IObservable<T2> in a far more convenient way than setting up ISubject<T1, T2>s which achieve the same thing.

Summary and advice

Go and play with it. Create a Subject, attach some IObservers, call subject.OnNext(new MyType) and watch your observers get called.

Don’t get obsessed with Rx.

Consider Rx when you’re not in control of the input stream; Rx solves “The data is here, what are you going to do about it?”, but not so much “I’ve finished processing this data, time to get some more”.

Be suspcious when you see async inside observers (Subscribe(async() => {})). Is it doing what you think it’s doing? Are you sure? Prove it.

Experiment with back-pressure until you understand where Rx makes it hard. This is especially important when you throw async in to the mix; what happens when something in your pipeline is async?