dotnet/reactive · error · InvalidOperationException

Accept should have called only one IObserver<T> method

Error message

Accept should have called only one IObserver<T> method

What it means

The same visitor in NotificationAsyncExtensions sets _valueTask when one of its IObserver<T> methods runs; if a second callback arrives (here OnCompleted after something already set _valueTask), it throws InvalidOperationException('Accept should have called only one IObserver<T> method'). The IObserver contract for Accept allows exactly one notification dispatch, so multiple calls indicate a malformed notification.

Solutions

1. Ensure the notification calls exactly one IObserver<T> method per Accept.
2. Use Notification<T>.CreateOnNext/OnError/OnCompleted instead of custom implementations.
3. Remove wrapper code that forwards Accept to more than one observer callback.
4. Fix test mocks to model one event per notification.

Example fix

// before (custom notification dispatching twice)
public void Accept(IObserver<T> observer) { observer.OnNext(_v); observer.OnCompleted(); }
// after
public void Accept(IObserver<T> observer) => observer.OnNext(_v);

if (notification is not Notification<T>) throw new InvalidOperationException("Only library-provided Notification<T> instances are supported by AcceptAsync");

static bool IsStandardNotification<T>(INotification<T> n) => n is Notification<T>;

try { await notification.AcceptAsync(asyncObserver); }
catch (InvalidOperationException ex) when (ex.Message.Contains("only one IObserver<T> method")) { /* multi-dispatch bug in notification — fix implementation */ }

When it happens

Trigger: Accepting an INotification<T> whose Accept (or visitor forwarding) invokes more than one observer method — e.g. OnNext followed by OnCompleted, or OnError called twice — from a custom or third-party notification implementation.

Common situations: Buggy custom notification types; double-dispatch in wrappers around AcceptAsync; test doubles that replay multiple events through one notification.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Accept did not call any IObserver<T> method
• Strings_Linq.NO_ELEMENTS
• Strings_Linq.NO_ELEMENTS
• error

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/93e780d1679d5d80. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/NotificationAsyncExtensions.cs:58

            {
                _asyncObserver = asyncObserver;
            }

            public async ValueTask Wait()
            {
                if (!_valueTask.HasValue)
                {
                    throw new InvalidOperationException("Accept did not call any IObserver<T> method");
                }

                await _valueTask.Value.ConfigureAwait(false);
            }

            public void OnCompleted()
            {
                if (_valueTask.HasValue)
                {
                    throw new InvalidOperationException("Accept should have called only one IObserver<T> method");
                }

                _valueTask = _asyncObserver.OnCompletedAsync();
            }

            public void OnError(Exception error)
            {
                if (_valueTask.HasValue)
                {
                    throw new InvalidOperationException("Accept should have called only one IObserver<T> method");
                }

                _valueTask = _asyncObserver.OnErrorAsync(error);
            }

            public void OnNext(T value)
            {
                if (_valueTask.HasValue)

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