dotnet/reactive · error · InvalidOperationException

The observer is currently processing a notification.

Error message

The observer is currently processing a notification.

What it means

TryEnter uses an Interlocked.CompareExchange state machine and detects re-entrancy: the observer is already Busy processing a previous OnNext/OnError/OnCompleted notification, so a nested or concurrent notification is refused.

Solutions

1. Await the previous notification's ValueTask before issuing another one from the same call site.
2. Serialize notifications through a queue (e.g. a subject) instead of calling observer methods from multiple contexts.
3. Design the observer so its handlers never synchronously call back into the observer.

When it happens

Trigger: Thrown at AsyncRx.NET/System.Reactive.Async/AsyncObserverBase.cs:76 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/208788987864b681. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/AsyncObserverBase.cs:76

            {
                return OnNextAsyncCore(value);
            }
            finally
            {
                Interlocked.Exchange(ref _status, Idle);
            }
        }

        protected abstract ValueTask OnNextAsyncCore(T value);

        private void TryEnter()
        {
            var old = Interlocked.CompareExchange(ref _status, Busy, Idle);

            switch (old)
            {
                case Busy:
                    throw new InvalidOperationException("The observer is currently processing a notification.");
                case Done:
                    throw new InvalidOperationException("The observer has already terminated.");
            }
        }
    }
}

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