OverrideViaProperty.cs

namespace System.Reactive.Linq
{
    /// <summary>
    /// Provides a set of static methods for writing in-memory queries over observable sequences.
    /// </summary>
    public static class ObservableEx
    {
        private static IQueryLanguageEx _override;

        /// <summary>
        /// Override the default implementation of operators.
        /// </summary>
        public static IQueryLanguageEx Override
        {
            get { return Volatile.Read(ref _override); }
            set { Interlocked.Exchange(ref _override, value);  }
        }

        [Experimental]
        public static IObservable<TResult> Create<TResult>(Func<IObserver<TResult>, IEnumerable<IObservable<object>>> iteratorMethod)
        {
            if (iteratorMethod == null)
                throw new ArgumentNullException(nameof(iteratorMethod));

            var ovr = Override;
            if (ovr != null)
            {
                return ovr.Create(iteratorMethod);
            }

            return new AnonymousObservable<TResult>(observer =>
                iteratorMethod(observer).Concat().Subscribe(_ => { }, observer.OnError, observer.OnCompleted));
        }
    }      
}