Detecting network failure with IAsyncResult

I am currently sending HTTP web requests from Unity to web server using C#.

I wish to make all the POST requests to communicate asynchronously.

I've followed MSDN Guideline to implement asynchronous network communication with the web server.

The problem is that I am having a hard time detecting server failures such as network timeout or connection establishment failure due to invalid URI.

Here are code snippets that I've implemented at the moment:

 protected void PostAsync(string uri, string jsonBody)
 {
 string fullUri = _uriPrefix + uri;
 HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(fullUri);
 ...
 request.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), request));
 }
 
 
 private void GetRequestStreamCallback(IAsyncResult asyncResult)
 {
 HttpWebRequest request = (HttpWebRequest)asyncResult.AsyncState;
 Stream requestStream = request.EndGetRequestStream(asyncResult);
 requestStream.Write(asyncState.inputStream, 0, asyncState.inputStream.Length);
 requestStream.Close();
 IAsyncResult result = (IAsyncResult)request.BeginGetResponse(new AsyncCallback(GetResponseCallback), request);
 }

When debugged using MonoDevelop, I found out that asyncResult parameter is of System.Net.WebAsyncResult instance which contains an exception indicating that the connection establishment was failed.

However, the problem is that I can't typecast the IAsyncResult asyncResult instance to System.Net.WebAsyncResult as such class does not seem to exist. (I've tried using System.net)

When continued, my debugger does not go beyond request.EndGetRequestStream(asyncResult) line. (Blocked)

Can anyone guide me on how I can detect connection establishment failure? Also, any other tips on how to handle such network failures would be greatly appreciated!

Thanks in advance,