rx-observables

Rx.NET Patterns and Practices

Patterns, anti-patterns, and decision guidance for System.Reactive (Rx.NET) in C#. Covers interface design, subject usage, subscription lifecycle, error handling, and testing.

Exposing Observables

Expose IObservable<T> as a property; never inherit from it.

Classes should not implement IObservable<T> directly. Expose a property instead, enabling consumers to compose operators before subscribing.

public class SensorReadings
{
    private readonly Subject<double> _readings = new();

    // Wrap with AsObservable() to prevent downcasting to Subject
    public IObservable<double> Readings => _readings.AsObservable();

    internal void Report(double value) => _readings.OnNext(value);
}

Key rules:

• Always call AsObservable() on subjects before exposing them. This prevents consumers from calling OnNext/OnError/OnCompleted.
• Never return null from a method or property typed IObservable<T>. Return Observable.Empty<T>() or Observable.Never<T>() instead.
• Avoid implementing IObservable<T> or IObserver<T> yourself. Use Observable.Create or subjects.

Subject Selection

Subjects are "mutable variables of the Rx world" (Erik Meijer). Use them only to generate a hot observable imperatively from a local source with no direct external observable to adapt.

Decision: Subject vs Observable.Create

Use Observable.Create when:

• Wrapping an external async/callback/event source into an observable
• Each subscriber needs independent state (cold observable)
• Cleanup logic ties naturally to the subscription's IDisposable

Use a Subject when:

• You need to imperatively push values from code you control (local source)
• Multiple subscribers must share the same live stream (hot observable)
• Modeling property-change or event-like notifications

If the source is an existing observable and you want hot behavior, use Publish/RefCount instead of piping through a subject.

Which Subject Type

BehaviorSubject<T> requires a seed value and immediately pushes it to new subscribers. Prefer it for "current state" scenarios.

ReplaySubject<T> can replay a bounded window (new ReplaySubject<T>(bufferSize)) or a time window. Always bound the buffer to avoid unbounded memory growth.

Subjects Must Be Private

Subjects are implementation details. Expose only IObservable<T> publicly:

private readonly BehaviorSubject<int> _count = new(0);
public IObservable<int> Count => _count.AsObservable();

Imperative Read-back Is a Design Smell

Exposing .Value / .Current from a BehaviorSubject<T> publicly introduces imperatively-accessed shared state, undermining the reactive model. Ben Lesh (RxJS lead): "using getValue() is a huge code smell... you're doing something imperative in declarative paradigm."

Acceptable: Internal read-modify-write within the owning class (e.g. _subject.OnNext(_subject.Value + 1)).

Avoid: Public .Value properties that let consumers pull state instead of subscribing.

Alternatives:

• Pass data as parameters to consumers instead of letting them read state.
• Use Scan within the observable chain for stateful accumulation.
• Use an OAPH-like pattern (subscribe internally, cache latest, expose read-only) when a synchronous read is genuinely necessary.

Hot vs Cold Observables

Cold: Each Subscribe triggers independent execution. Factory methods like Observable.Create, Observable.Defer, Observable.Timer produce cold observables.

Hot: All subscribers share a single underlying source. Subjects are inherently hot. Use Publish/RefCount to share a cold source:

IObservable<long> shared = Observable
    .Interval(TimeSpan.FromSeconds(1))
    .Publish()
    .RefCount();

Publish().RefCount() connects on first subscriber and disconnects when the last unsubscribes. Use Publish().AutoConnect(n) if you need to wait for n subscribers before connecting.

Subscription Lifecycle

Every Subscribe returns an IDisposable. Failing to dispose leaks subscriptions and can leak the subscriber (prevented from GC by the observable's reference to it).

Disposable Types

Patterns

Collect subscriptions with CompositeDisposable and dispose in one call:

private readonly CompositeDisposable _subscriptions = new();

public void Initialize()
{
    source1.Subscribe(HandleItem1).DisposeWith(_subscriptions);
    source2.Subscribe(HandleItem2).DisposeWith(_subscriptions);
}

public void Dispose() => _subscriptions.Dispose();

DisposeWith is in the System.Reactive.Disposables.Fluent namespace (available since Rx.NET 6.1).

Use SerialDisposable when replacing one subscription with another:

private readonly SerialDisposable _current = new();

public void SwitchSource(IObservable<int> newSource)
{
    // Disposes previous subscription automatically
    _current.Disposable = newSource.Subscribe(Handle);
}

Only dispose subscriptions early if you need to unsubscribe before the observable completes. Finite sequences (Observable.Return, .Take, etc.) clean up on completion.

Error Handling

An OnError notification terminates the sequence. Subscribers that don't provide an OnError handler will have the exception thrown on the calling thread.

Always provide an OnError handler in Subscribe.

Operators

Prefer typed Catch<TSource, TException> over untyped variants to avoid swallowing unexpected exceptions:

IObservable<string> resilient = source
    .Catch<string, TimeoutException>(_ => Observable.Empty<string>());

Avoid: Putting side effects in Select/Where callbacks. Use Do for explicit side effects before operators that may error.

Thread Safety

The Rx contract requires that OnNext, OnError, and OnCompleted calls are serialized (non-concurrent, non-overlapping). Subjects do NOT enforce this internally. Concurrent OnNext calls can produce out-of-order delivery or corrupt operator state.

If multiple threads may call OnNext:

// Wrap with Synchronize to enforce sequential delivery
var safe = subject.Synchronize();

Or use Observable.Create which naturally scopes to a single subscriber.

Never call OnNext concurrently on a subject. This violates the Rx contract and causes subtle threading bugs in downstream operators.

ObserveOn(scheduler) and SubscribeOn(scheduler) control which thread notifications are delivered on. Place ObserveOn/SubscribeOn immediately before Subscribe, not buried mid-chain.

Testing

Use Microsoft.Reactive.Testing NuGet package for TestScheduler.

TestScheduler

TestScheduler virtualizes time, allowing time-dependent tests to run in microseconds instead of real time:

var scheduler = new TestScheduler();

// Advance virtual time instead of waiting real seconds
var results = scheduler.Start(() =>
    Observable.Interval(TimeSpan.FromSeconds(1), scheduler).Take(3)
);

Assert.That(results.Messages, Has.Count.EqualTo(4)); // 3 OnNext + 1 OnCompleted

Key methods:

• AdvanceBy(ticks): Move clock forward by relative amount.
• AdvanceTo(ticks): Move clock to absolute time.
• Start(create): Creates, subscribes, disposes; returns recorded messages.

One tick = 100ns. Use TimeSpan.FromSeconds(n).Ticks for readability.

Inject Schedulers

Accept IScheduler as a parameter in production code so tests can supply TestScheduler:

public IObservable<T> PollEvery<T>(
    TimeSpan interval,
    Func<T> fetch,
    IScheduler scheduler)
{
    return Observable.Interval(interval, scheduler).Select(_ => fetch());
}

Testing Without TestScheduler

For simpler cases, subscribe and collect values into a list:

var results = new List<int>();
source.Subscribe(results.Add);
// ... trigger emissions ...
Assert.That(results, Is.EqualTo(new[] { 1, 2, 3 }));

Use Subject<T> in tests as a controllable source to push values imperatively. This is one of the legitimate uses of subjects.

Common Anti-patterns

1. Implementing IObservable<T> on a class. Use composition (expose a property) not inheritance.
2. Exposing Subject<T> publicly. Always wrap with AsObservable(). Public subjects let anyone call OnNext or OnCompleted on your stream.
3. Using getValue()/.Value publicly. Indicates imperative thinking in a reactive codebase. Subscribe to the stream instead.
4. Subscribing without OnError. Unhandled OnError throws. Always handle errors.
5. Forgetting to dispose subscriptions. Leaked subscriptions cause memory leaks and unexpected side effects.
6. Concurrent OnNext on subjects. Violates Rx contract. Use Synchronize() or restructure to avoid concurrent calls.
7. Blocking operators (First, Last, Single). These block the calling thread. Use async alternatives (FirstAsync) or test-specific patterns.
8. Side effects in Select/Where. Use Do for explicit side effects. Keep Select and Where pure.
9. Unbounded ReplaySubject. Always specify a buffer size or time window to prevent unbounded memory growth.
10. Using a subject to bridge an existing observable. Use Publish, Replay, or other sharing operators instead of subscribing to one observable and forwarding into a subject.

Quick Reference

Need to...                          Use...
------------------------------------+-------------------------------------
Wrap event/callback/async -> Rx     Observable.Create / Observable.FromEvent
Push values imperatively (local)    Subject<T> (private, expose AsObservable)
Current-value property semantics    BehaviorSubject<T>
Share a cold observable             Publish().RefCount()
Group subscription disposal         CompositeDisposable + DisposeWith
Replace subscriptions               SerialDisposable
Handle errors in chain              Catch<TSource, TException>
Retry transient failures            Retry(count)
Control time in tests               TestScheduler (Microsoft.Reactive.Testing)
Enforce sequential OnNext           subject.Synchronize()