gistfile1.cs

IObservable<KeyValuePair<string, bool>> ValidateUriAsync(string Uri)
{
    var request = WebRequest.Create(Uri);

    // This gives us a "GetResponseAsync" function whose prototype is
    // IObservable[WebResponse] GetResponseAsync() - just like the sync
    // version, but the return value wrapped in IObservable. 
    var response_fetcher = Observable.FromAsyncPattern<WebResponse>(
        request.BeginGetResponse, request.EndGetResponse);

    // The clever part here is the Catch - if the response_fetcher IObservable
    // ends with OnError, we'll instead act as if it didn't end and splice in
    // a "completed successfully" IObservable who returns null and ends
    return response_fetcher()
        .Catch(Observable.Return<WebResponse>(null))
        .Select(resp => new KeyValuePair<string, bool>(Uri, resp != null && resp.StatusCode == HttpStatusCode.OK));
}