RxError.cs

using System;
using System.Net.Http;
using System.Reactive.Linq;
using System.Reactive.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        new HttpClient().GetStringAsync("http://twitter.com/statuses/user_timeline.json")
            .ToObservable()
            .Catch((HttpRequestException e) => Observable.Return("hogehoge"))
            .Subscribe(Console.WriteLine);

        Console.ReadLine();
    }
}