What causes "Strings_Linq.NO_ELEMENTS" in dotnet/reactive?

Use the nullable overload Observable.Average(source.Select(x => (int?)x)) which yields null for empty sequences

dotnet/reactive · error · InvalidOperationException

Strings_Linq.NO_ELEMENTS

Error message

Strings_Linq.NO_ELEMENTS

What it means

An Average operator (integer variant) throws InvalidOperationException with Strings_Linq.NO_ELEMENTS when the source completes without emitting any value, because the average of an empty sequence is undefined. The operator forwards the error on OnCompleted instead of producing a result.

Solutions

Use the nullable overload Observable.Average(source.Select(x => (int?)x)) which yields null for empty sequences
Catch the error: source.Average().Catch(Observable.Return(0.0)) or supply a default via Materialize/Dematerialize
Ensure the source emits at least one element before completing

Example fix

// before
source.Average().Subscribe(avg => ...); // throws on empty
// after
source.Select(x => (int?)x).Average().Subscribe(avg => HandleAvg(avg ?? 0));

Defensive patterns

Strategy: try-catch

Validate before calling

bool isEmpty = true; source.Subscribe(_ => isEmpty = false, () => { if (isEmpty) avgSubject.OnNext(0); });

Try / catch

source.Average()
    .Catch<double>(ex => ex is InvalidOperationException ? Observable.Return(0.0) : Observable.Throw<double>(ex))
    .Subscribe(avg => ...);

Prevention

Use the nullable-element Average overload when the source may be empty
Validate that sources emit data before completion in integration tests
Supply sensible defaults (0, null) for empty aggregates at the call site

When it happens

Trigger: Subscribing to Observable.Average over an empty source: an empty int/long array, a stream fully filtered out, or a completed subject with no OnNext.

Common situations: Averaging sensor or metrics streams that produced no samples; averaging query results that came back empty; tests that forgot to push values before completion.

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/8c85dc22d8e73172. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable/Average.cs:59

                }
                catch (Exception ex)
                {
                    ForwardOnError(ex);
                }
            }

            public override void OnCompleted()
            {
                if (_count > 0)
                {
                    ForwardOnNext(_sum / _count);
                    ForwardOnCompleted();
                }
                else
                {
                    try
                    {
                        throw new InvalidOperationException(Strings_Linq.NO_ELEMENTS);
                    }
                    catch (Exception e)
                    {
                        ForwardOnError(e);
                    }
                }
            }
        }
    }

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

        public AverageSingle(IObservable<float> source)
        {
            _source = source;
        }

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