dotnet/reactive · error · InvalidOperationException

NO_ELEMENTS

Error message

NO_ELEMENTS

What it means

SingleAsync also requires at least one element: if OnCompleted arrives with no value ever seen, it throws InvalidOperationException(Strings_Linq.NO_ELEMENTS) forwarded via OnError. This matches LINQ Single() throwing on an empty sequence.

Solutions

1. Guarantee the source emits exactly one element before SingleAsync.
2. Use SingleOrDefaultAsync when zero elements is an expected, acceptable outcome.
3. Catch InvalidOperationException in OnError and supply a default value.

Example fix

// before
var user = await users.Where(u => u.Id == id).SingleAsync();
// after
var user = await users.Where(u => u.Id == id).SingleOrDefaultAsync();

try { var v = await source.SingleAsync(); }
catch (InvalidOperationException) { var v = default(TSource); }

When it happens

Trigger: source.SingleAsync() on a stream that completes without emitting any OnNext (e.g. Observable.Empty<T>().SingleAsync()).

Common situations: Filtering with a predicate that matches nothing; querying an entity by id that was deleted; subscribing before data is written and the source closes.

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

• MORE_THAN_ONE_ELEMENT
• Source sequence doesn't contain any elements.
• Element no longer available in the buffer.
• Strings_Linq.NO_ELEMENTS
• Strings_Linq.NO_ELEMENTS

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/eef383c3280587c1. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable/SingleAsync.cs:57

                        }
                        catch (Exception e)
                        {
                            ForwardOnError(e);
                        }
                        return;
                    }

                    _value = value;
                    _seenValue = true;
                }

                public override void OnCompleted()
                {
                    if (!_seenValue)
                    {
                        try
                        {
                            throw new InvalidOperationException(Strings_Linq.NO_ELEMENTS);
                        }
                        catch (Exception e)
                        {
                            ForwardOnError(e);
                        }
                    }
                    else
                    {
                        ForwardOnNext(_value!);
                        ForwardOnCompleted();
                    }
                }
            }
        }

        internal sealed class Predicate : Producer<TSource, Predicate._>
        {
            private readonly IObservable<TSource> _source;

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