dotnet/reactive · error · InvalidOperationException

Sequence contains no elements

Error message

Sequence contains no elements

What it means

AsyncSubject.GetResult (the task-like GetResult used by await and by blocking waiters) throws InvalidOperationException("Sequence contains no elements") when the subject completed without ever producing a value and without an error. An AsyncSubject only yields a value if OnNext was called at least once before OnCompleted.

Solutions

  1. Guard with await subject.FirstOrDefaultAsync() or use .DefaultIfEmpty() before awaiting.
  2. Check subject.Has.Value pattern or subscribe with a default in your pipeline.
  3. If empty results are valid, catch InvalidOperationException around GetResult/await.
  4. Fix the upstream source so it emits at least one value before completion.

Example fix

// before
var value = subject.GetResult();
// after
var value = await subject.DefaultIfEmpty(defaultValue).FirstAsync();
Defensive patterns

Strategy: try-catch

Validate before calling

var hasValue = await subject.AnyAsync();
if (!hasValue) value = defaultValue; else value = await subject.FirstAsync();

Try / catch

try { var value = subject.GetResult(); }
catch (InvalidOperationException) { var value = defaultValue; }

Prevention

When it happens

Trigger: Awaiting or calling GetResult on an AsyncSubject<T> that received OnCompleted with zero OnNext calls (or with only OnError, that path throws the stored exception instead).

Common situations: A source sequence that legitimately emitted nothing; a filtered/empty source; a race where GetResult runs before any value arrived but after completion flag read.

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

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/38bcf3ca0a0b5075. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Subjects/AsyncSubject.cs:434

        public T GetResult()
        {
            if (Volatile.Read(ref _observers) != Terminated)
            {
                using var e = new ManualResetEventSlim(initialState: false);

                //
                // [OK] Use of unsafe Subscribe: this type's Subscribe implementation is safe.
                //
                Subscribe/*Unsafe*/(new BlockingObserver(e));

                e.Wait();
            }

            _exception?.Throw();

            if (!_hasValue)
            {
                throw new InvalidOperationException(Strings_Linq.NO_ELEMENTS);
            }

            return _value!;
        }

        private sealed class BlockingObserver : IObserver<T>
        {
            private readonly ManualResetEventSlim _e;

            public BlockingObserver(ManualResetEventSlim e) => _e = e;

            public void OnCompleted() => Done();

            public void OnError(Exception error) => Done();

            public void OnNext(T value) { }

            private void Done() => _e.Set();

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