Scan, Aggregate, Do and DistinctUntilChanged throw callback exceptions to the producer instead of delivering OnError

Start in src/Primitives.Shared/SignalOperatorParityMixins.RxNames.cs, then trace the forwarding calls to FoldSignal, ReduceSignal, TapSignal, UniqueSignal and UniqueBySignal; compare their callback handling with Select and Where. Add or locate tests for callback exceptions and downstream observer exceptions. Done means callback exceptions reach OnError and stop the subscription, while observer exceptions still reach the producer.

Summary

Some operators handle an exception thrown by a user callback differently from Select and Where. Select and Where deliver it to the subscriber as OnError and end the subscription, as System.Reactive does. Scan, Aggregate, Do, DistinctUntilChanged(comparer) and DistinctUntilChangedBy do not. They throw the exception to the code that called the source's OnNext, the subscriber never gets OnError, and the subscription stays active, so the callback runs again on the next value.

These operators are the System.Reactive-named parity aliases, so code written against Rx expects the Rx behaviour.

Repro (ReactiveUI.Primitives 9.0.0, net10.0)

using ReactiveUI.Primitives;
using ReactiveUI.Primitives.Signals;

static void Probe(string name, Func<IObservable<int>, IObservable<int>> op)
{
    var subject = new Signal<int>();
    string outcome = "nothing";
    op(subject).Subscribe(v => outcome = "OnNext " + v, e => outcome = "OnError " + e.Message, () => outcome = "OnCompleted");
    try { subject.OnNext(1); }
    catch (Exception e) { outcome = "thrown to OnNext caller: " + e.Message + " (subscriber saw " + outcome + ")"; }

    string after;
    try { subject.OnNext(2); after = outcome; } catch (Exception e) { after = "second OnNext threw " + e.Message; }
    Console.WriteLine($"{name,-38} {outcome} | after: {after}");
}

var boom = new InvalidOperationException("boom");
Probe("Select(selector throws)", s => s.Select<int, int>(_ => throw boom));
Probe("Where(predicate throws)", s => s.Where(_ => throw boom));
Probe("Scan(accumulator throws)", s => s.Scan(0, (_, _) => throw boom));
Probe("Aggregate(accumulator throws)", s => s.Aggregate(0, (_, _) => throw boom));
Probe("Do(onNext throws)", s => s.Do(_ => throw boom));
Probe("DistinctUntilChanged(comparer throws)", s => s.DistinctUntilChanged(new ThrowingComparer()));
Probe("DistinctUntilChangedBy(key throws)", s => s.DistinctUntilChangedBy<int, int>(_ => throw boom));

sealed class ThrowingComparer : IEqualityComparer<int>
{
    public bool Equals(int x, int y) => throw new InvalidOperationException("boom");
    public int GetHashCode(int obj) => throw new InvalidOperationException("boom");
}

Output:

Select(selector throws)                OnError boom | after: OnError boom
Where(predicate throws)                OnError boom | after: OnError boom
Scan(accumulator throws)               thrown to OnNext caller: boom (subscriber saw nothing) | after: second OnNext threw boom
Aggregate(accumulator throws)          thrown to OnNext caller: boom (subscriber saw nothing) | after: second OnNext threw boom
Do(onNext throws)                      thrown to OnNext caller: boom (subscriber saw nothing) | after: second OnNext threw boom
DistinctUntilChanged(comparer throws)  OnNext 1 | after: second OnNext threw boom
DistinctUntilChangedBy(key throws)     thrown to OnNext caller: boom (subscriber saw nothing) | after: second OnNext threw boom

DistinctUntilChanged emits the first value because it doesn't compare it to anything; the comparer exception happens on the second value.

Rx comparison

In System.Reactive, Scan, Aggregate, Do, DistinctUntilChanged(comparer) and DistinctUntilChanged(keySelector) all catch exceptions from the user callback (accumulator, onNext action, comparer, key selector). They pass the exception to the downstream observer's OnError and stop. That is what this library's Select and Where already do.

Location

The aliases are in src/Primitives.Shared/SignalOperatorParityMixins.RxNames.cs. They forward to FoldSignal (Scan), ReduceSignal (Aggregate), TapSignal (Do), UniqueSignal (DistinctUntilChanged) and UniqueBySignal (DistinctUntilChangedBy). Those implementations call the callback without the try/OnError handling that Select/Where use.

Suggested fix

Wrap the callback calls in these operators the same way Select/Where do. Catch the exception, mark the operator stopped, and deliver OnError downstream. Exceptions thrown by the downstream observer itself should still go to the producer, not be turned into OnError.

Context

Runic-Artifex's DynamicData fork for ReactiveUI.Primitives currently ships its own replacements for Scan, Do and DistinctUntilChanged to get the Rx behaviour.