namespace System.Reactive.Linq.ObservableImpl { internal sealed class DefaultIfEmpty<TSource> : Producer<TSource, DefaultIfEmpty<TSource>._> { internal sealed class _ : IdentitySink<TSource> { private readonly TSource _defaultValue; private bool _found; public _(TSource defaultValue, IObserver<TSource> observer) : base(observer) { _defaultValue = defaultValue; _found = false; } public override void OnNext(TSource value) { _found = true; ForwardOnNext(value); } public override void OnCompleted() { if (!_found) ForwardOnNext(_defaultValue); ForwardOnCompleted(); } } 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) { return new _(_defaultValue, observer); } protected override void Run(_ sink) { sink.Run(_source); } } }