namespace System.Reactive.Linq.ObservableImpl { internal sealed class DefaultIfEmpty<TSource> : Producer<TSource, DefaultIfEmpty<TSource>._> { internal sealed class _ : Sink<TSource>, IObserver<TSource> { private readonly TSource _defaultValue; private bool _found; public _(TSource defaultValue, IObserver<TSource> observer, IDisposable cancel) : base(observer, cancel) { _defaultValue = defaultValue; _found = false; } public void OnNext(TSource value) { _found = true; _observer.OnNext(value); } public void OnError(Exception error) { _observer.OnError(error); base.Dispose(); } public void OnCompleted() { if (!_found) _observer.OnNext(_defaultValue); _observer.OnCompleted(); base.Dispose(); } } private readonly IObservable<TSource> _source; private readonly TSource _defaultValue; public DefaultIfEmpty(IObservable<TSource> source, TSource defaultValue) { _source = source; _defaultValue = defaultValue; } protected override _ CreateSink(IObserver<TSource> observer, IDisposable cancel) { return new _(_defaultValue, observer, cancel); } protected override IDisposable Run(_ sink) { return ObservableExtensions.SubscribeSafe<TSource>(_source, (IObserver<TSource>)sink); } } }