Question

I found an example about HTTP POST in msdn, but I am wondering how can I make use of reactive extensions here.

using System;
 using System.Net;
 using System.IO;
 using System.Text; using System.Threading;

class HttpWebRequestBeginGetRequest
 {
     private static ManualResetEvent allDone = new ManualResetEvent(false);

    public static void Main(string[] args)
     {


        // Create a new HttpWebRequest object.
         HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://www.contoso.com/example.aspx");

        request.ContentType = "application/x-www-form-urlencoded";

        // Set the Method property to 'POST' to post data to the URI.
         request.Method = "POST";

        // start the asynchronous operation
         request.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), request);

        // Keep the main thread from continuing while the asynchronous
         // operation completes. A real world application
         // could do something useful such as updating its user interface. 
        allDone.WaitOne();
     }

    private static void GetRequestStreamCallback(IAsyncResult asynchronousResult)
     {
         HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;

        // End the operation
         Stream postStream = request.EndGetRequestStream(asynchronousResult);

        Console.WriteLine("Please enter the input data to be posted:");
         string postData = Console.ReadLine();

        // Convert the string into a byte array.
         byte[] byteArray = Encoding.UTF8.GetBytes(postData);

        // Write to the request stream.
         postStream.Write(byteArray, 0, postData.Length);
         postStream.Close();

        // Start the asynchronous operation to get the response
         request.BeginGetResponse(new AsyncCallback(GetResponseCallback), request);
     }

    private static void GetResponseCallback(IAsyncResult asynchronousResult)
     {
         HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;

        // End the operation
         HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult);
         Stream streamResponse = response.GetResponseStream();
         StreamReader streamRead = new StreamReader(streamResponse);
         string responseString = streamRead.ReadToEnd();
         Console.WriteLine(responseString);
         // Close the stream object
         streamResponse.Close();
         streamRead.Close();

        // Release the HttpWebResponse
         response.Close();
         allDone.Set();
     }
 }

I am trying to use the following code, but it does not work. Can anyone help me out on this?
Thanks in advance -Peng

    return (from request in
                Observable.Return((HttpWebRequest)WebRequest.Create(new Uri(postUrl))).Catch(Observable.Empty<HttpWebRequest>())
                .Do(req =>
                        {
                            // Set up the request properties
                            req.Method = "POST";
                            req.ContentType = contentType;
                            req.UserAgent = userAgent;
                            req.CookieContainer = new CookieContainer();
                            Observable.FromAsyncPattern<Stream>(req.BeginGetRequestStream, req.EndGetRequestStream)()
                                .ObserveOnDispatcher()
                                .Subscribe(stream =>
                                        {
                                            stream.Write(formData, 0,
                                                         formData.Length);
                                            stream.Close();

                                        })
                                ;

                        })
            from response in
                Observable.FromAsyncPattern<WebResponse>(request.BeginGetResponse, request.EndGetResponse)().Catch(Observable.Empty<WebResponse>())
            from item in GetPostResponse(response.GetResponseStream()).ToObservable().Catch(Observable.Empty<string>())
            select item).ObserveOnDispatcher();

Edit: To make it clear, I want to use the rx to implement the same logic in MSDN example.
in the MSDN example, it seems it first makes async call to write RequestStream, and then in the GetRequestStreamCallback, fires another async call to get the response.
Using Rx, I am able to create 2 observables
1. Observable.FromAsyncPattern(request.BeginGetRequestStream, request.EndGetRequestStream)()
2. Observable.FromAsyncPattern(request.BeginGetResponse, request.EndGetResponse)()
The problem is the second observable depends on the first one’s result, so how can I do this in Rx?
In the first observable’s subcribe method to create the seond observable? is it the good way?

Answer 1

This is how I am doing it. I configure the two Async patters up front, then use SelectMany to chain them together.
I have cut out the error handling etc from this code to keep it simple and show only the bare minimum to get it working. You should append a .Catch() similar to your own code, and if you want to get more than just a string out (say the response code) then you’ll need to create a class/struct to hold all the bits of data you need and return that instead.

public IObservable<string> BeginPost(Uri uri, string postData) {
  var request = HttpWebRequest.CreateHttp(uri);
  request.Method = "POST";
  request.ContentType = "application/x-www-form-urlencoded";

  var fetchRequestStream = Observable.FromAsyncPattern<Stream>(request.BeginGetRequestStream, request.EndGetRequestStream);
  var fetchResponse = Observable.FromAsyncPattern<WebResponse>(request.BeginGetResponse, request.EndGetResponse);
  return fetchRequestStream().SelectMany(stream => {
    using (var writer = new StreamWriter(stream)) writer.Write(postData);
    return fetchResponse();
  }).Select(result => {
    var response = (HttpWebResponse)result;
    string s = ""; 
    if (response.StatusCode == HttpStatusCode.OK) { 
      using (var reader = new StreamReader(response.GetResponseStream())) s = reader.ReadToEnd(); 
    }
    return s;
  });
}