gistfile1.cs

public static IObservable<RecoveryOptionResult> Throw(UserError error)
{
    var handlers = (overriddenRegisteredUserErrorHandlers != null) ?
        new[] { overriddenRegisteredUserErrorHandlers } :
        registeredUserErrorHandlers.ToArray().Reverse();

    // NB: This is a little complicated - here's the idea: we have a 
    // list of handlers that we're running down *in order*. If we find
    // one that doesn't return null, we're going to return this as an 
    // Observable with one item (the result).
    //
    // If *none* of the handlers are interested in this UserError, we're
    // going to OnError the Observable.
    var ret = handlers.ToObservable()
        .Select(handler => Observable.Start(() => handler(error), RxApp.DeferredScheduler))
        .Concat()
        .Where(x => x != null).Select(x => x.Value)
        .Concat(Observable.Throw<RecoveryOptionResult>(new UnhandledUserErrorException(error)))
        .Take(1)
        .Multicast(new AsyncSubject<RecoveryOptionResult>());

    ret.Connect();
    return ret;
}