twittersend.cs

public static IObservable<Stream> GetRequestStreamAsync(this WebRequest request)
{
    return Observable.FromAsyncPattern<Stream>(request.BeginGetRequestStream, request.EndGetRequestStream)();
}

public static IObservable<WebResponse> GetResponseAsync(this WebRequest request)
{
    return Observable.FromAsyncPattern <WebResponse>(request.BeginGetResponse, request.EndGetResponse)();
}

public static IObservable<Stream> GetResponseStreamAsync(this WebRequest request)
{
    return Observable.FromAsyncPattern <Stream>(request.BeginGetRequestStream, request.EndGetRequestStream)();
}

public static IObservable<Unit> WriteAsync(this Stream stream, byte[] buffer, int offset, int count)
{
    return Observable.FromAsyncPattern<byte[], int, int>(
        stream.BeginWrite,
        stream.EndWrite)(buffer, offset, count);
}

private static IObservable<WebRequest> CreateWebRequest(string url, string userName, string password, string method)
{
    return Observable.Create<WebRequest>(observer =>
     {
         try
         {
             var request = (HttpWebRequest)WebRequest.Create(url);
             request.Credentials = new NetworkCredential(userName, password);
             request.Method = method;
             observer.OnNext(request);
             observer.OnCompleted();
         }
         catch (Exception exception)
         {
             observer.OnError(exception);
         }

         return () => { };
     });
}

var tweetObservalue =
    from request in CreateWebRequest("http://twitter.com/statuses/update.xml", userName, password, "POST")
    from requestStream in request.GetRequestStreamAsync()
    let tweet = HttpUtility.HtmlEncode("status=" + text)
    let buffer = Encoding.UTF8.GetBytes(tweet)
    from _ in requestStream.WriteAsync(buffer, 0, buffer.Length)
    from response in request.GetResponseAsync()
    let responseStream = response.GetResponseStream()
    let reader = new StreamReader(responseStream)
    select reader.ReadToEnd();

tweetObservalue.Subscribe(tweet => MessageBox.Show(tweet));