Silverlight HttpWebRequest内容长度为空,WTF?

当我运行以下代码时,接收请求的RESTful web服务有一个空的主体(内容长度=0),我不知道为什么?

        var request = (HttpWebRequest)WebRequest.Create(uri);
        request.ContentType = "text/xml";
        request.Method = "POST";
        request.CookieContainer = new CookieContainer();
        request.CookieContainer.Add(uri, new Cookie("SSOToken", HttpUtility.UrlEncode(SsoToken)));
        request.ContentLength = data.Length;

        request.BeginGetRequestStream(ar1 =>
        {
            var byteArray = Encoding.UTF8.GetBytes(data);

            var stream = request.EndGetRequestStream(ar1);
            stream.Write(byteArray, 0, data.Length);
            stream.Close();

            request.BeginGetResponse(ar2 => HandleSearchCompleted(ar2, request, action), state);
        }, state);