How to set the X-RequestDigest Value ?

Hi,

I need to post data in sharepoint with REST requests and C#. To do that I have to set the X-RequestDigest Value. But I don't know how ? The  HttpWebRequest class doesn't seem to have a property for that. I tried to do this :

HttpWebRequest request.Headers.Add("X-RequestDigest", digestValue);

Here "digestValue" is a string which contains the form digest value. It does'nt work, I get a webException : 500 Internal server error.

This is my request code :

 public HttpWebResponse RequestPost(string oDataUrl, string data)
        {    
            byte[] byteArray = Encoding.ASCII.GetBytes(data);

            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(sharepointUrl.ToString() + oDataUrl);
            request.Method = "POST";
            request.Accept = "application/atom+xml";
            request.ContentLength = byteArray.Length;
            request.ContentType = "application/atom+xml;type=entry";
            request.Headers.Add("Authorization", "Bearer " + accessToken);

            if (digestValue != null) {

                request.Headers.Add("X-RequestDigest", digestValue);
            }

            if (request.ContentLength > 0)
            {
                Stream stream = request.GetRequestStream();
                stream.Write(byteArray, 0, byteArray.Length);
                stream.Close();
            }
            return (HttpWebResponse)request.GetResponse();
        }

Thanks.