dotnet/reactive · error · InvalidOperationException

The subject has no value.

Error message

The subject has no value.

What it means

AsyncAsyncSubject<T>.GetResult blocks/awaits until the subject completes and then returns the last emitted value. If the subject completed without ever receiving a value (only OnCompleted, or an error was already rethrown), there is no value to return and the subject throws InvalidOperationException with the message 'The subject has no value.'

Solutions

1. Ensure the producer calls OnNext at least once before OnCompleted
2. Check _hasValue semantics: only await/GetResult when the pipeline guarantees a value
3. Handle the empty-completion case explicitly (e.g. a default value) instead of calling GetResult

Example fix

// before
var value = await subject; // throws if subject completed empty
// after
var value = hasPublishedValue ? await subject : defaultValue;

if (subject.CompletionStatus == AsyncSubjectStatus.CompletedWithoutValue) useDefaultInstead();

bool HasResult(AsyncAsyncSubject<T> s) => s.HasReceivedValue;

try { value = await subject; } catch (InvalidOperationException ex) when (ex.Message == "The subject has no value.") { value = defaultValue; }

When it happens

Trigger: Calling GetResult on an AsyncAsyncSubject that completed via OnCompleted without any OnNext, i.e. _hasValue is false and _error is null.

Common situations: Awaiting an async subject that a producer completed without publishing; race where the producer finished early due to a cancellation or empty branch of a pipeline.

Understand the failure class

Background: EmptyResultError / "no results found": when an API or scraper succeeds but returns zero rows — this error's family across 9 libraries.

Related errors

• AdvanceTo cannot be called when the scheduler is already…
• AdvanceBy cannot be called when the scheduler is already…
• subject
• error
• error

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/5f24253a686b9fe4. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Subjects/AsyncAsyncSubject.cs:175

        public T GetResult()
        {
            if (!IsCompleted)
            {
                var e = new ManualResetEventSlim(initialState: false);

                OnCompleted(() => { e.Set(); }, originalContext: false);

                e.Wait();
            }

            if (_error != null)
            {
                ExceptionDispatchInfo.Capture(_error).Throw();
            }

            if (!_hasValue)
            {
                throw new InvalidOperationException("The subject has no value.");
            }

            return _value;
        }

        public void OnCompleted(Action continuation)
        {
            if (continuation == null)
                throw new ArgumentNullException(nameof(continuation));

            OnCompleted(continuation, originalContext: true);
        }

        private void OnCompleted(Action continuation, bool originalContext)
        {
            var subscribeTask = SubscribeAsync(new AwaitObserver(continuation, originalContext));

            subscribeTask.AsTask().ContinueWith(t =>

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