namespace System.Reactive.Linq.ObservableImpl { internal sealed class ElementAt<TSource> : Producer<TSource, ElementAt<TSource>._> { internal sealed class _ : IdentitySink<TSource> { private int _i; public _(int index, IObserver<TSource> observer) : base(observer) { _i = index; } public override void OnNext(TSource value) { if (_i == 0) { ForwardOnNext(value); ForwardOnCompleted(); } _i--; } public override void OnCompleted() { if (_i >= 0) ForwardOnError(new ArgumentOutOfRangeException("index")); } } 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) { return new _(_index, observer); } protected override void Run(_ sink) { sink.Run(_source); } } }