C# REST client sending data using POST

I'm trying to send a simple POST request to a REST web service and print the response (code is below, mostly taken from Yahoo! developer documentation and the MSDN code snippets provided with some of the documentation). I would expect the client to send:

Request Method: POST (i.e. I expect $_SERVER['REQUEST_METHOD'] == 'POST' in PHP)
Data: foo=bar (i.e. $_POST['foo'] == 'bar' in PHP)

However, it seems to be sending:

Request Method: FOO=BARPOST
Data: (blank)

I know the API works as I've tested it with clients written in Python and PHP, so I'm pretty sure it must be a problem with my C#. I'm not a .NET programmer by trade so would appreciate any comments/pointers on how to figure out what the problem is - I'm sure it's something trivial but I can't spot it myself.

uri, user and password variables are set earlier in the code - they work fine with GET requests.

request = (HttpWebRequest) WebRequest.Create(uri);
request.Credentials = new NetworkCredential(user, password);
request.Method = WebRequestMethods.Http.Post;
request.ContentType = "application/x-www-form-urlencoded";

string postData = "foo=bar";
request.ContentLength = postData.Length;

StreamWriter postStream = new StreamWriter(request.GetRequestStream(), System.Text.Encoding.ASCII);
postStream.Write(postData);
postStream.Close();

response = (HttpWebResponse) request.GetResponse();

The REST API is written in PHP, and the $_POST array is empty on the server when using the C# client.

Eventually found the HttpWebRequest.PreAuthenticate property which seems to solve the problem if the code is edited like so:

request = (HttpWebRequest) WebRequest.Create(uri);
request.PreAuthenticate = true;
request.Credentials = new NetworkCredential(user, password);
request.Method = WebRequestMethods.Http.Post;

From the documentation I presume this forces authentication before the actual POST request is sent. I'm not sure why the class doesn't do this automatically (libraries for other languages make this process transparent, unless you explicitly turn it off), but it has solved the problem for me and may save someone else another 2 days of searching and hair-pulling.

For what it's worth, PreAuthenticate doesn't need to be set for GET requests, only POST, although if you do set it for a GET request everything will still work, but take slightly longer.