namespace System.Reactive.Linq.ObservableImpl { internal sealed class ElementAt<TSource> : Producer<TSource, ElementAt<TSource>._> { internal sealed class _ : Sink<TSource>, IObserver<TSource> { private int _i; public _(int index, IObserver<TSource> observer, IDisposable cancel) : base(observer, cancel) { _i = index; } public void OnNext(TSource value) { if (_i == 0) { _observer.OnNext(value); _observer.OnCompleted(); base.Dispose(); } _i--; } public void OnError(Exception error) { _observer.OnError(error); base.Dispose(); } public void OnCompleted() { _observer.OnError(new ArgumentOutOfRangeException("index")); base.Dispose(); } } private readonly IObservable<TSource> _source; private readonly int _index; public ElementAt(IObservable<TSource> source, int index) { _source = source; _index = index; } protected override _ CreateSink(IObserver<TSource> observer, IDisposable cancel) { return new _(_index, observer, cancel); } protected override IDisposable Run(_ sink) { return ObservableExtensions.SubscribeSafe<TSource>(_source, (IObserver<TSource>)sink); } } }