stream.length' выдал исключение типа 'system.notsupportedexception'

"stream.length' threw an exception of type 'system.notsupportedexception'"

protected void Button1_Click(object sender, EventArgs e)
{
    string encodedURL = "http://abc.com/psms/servlet/psms.Eservice2";

    HttpWebRequest objRequest = (HttpWebRequest)WebRequest.Create(encodedURL);

    try
    {
        string xmlstr = "data=<!DOCTYPE REQUESTCREDIT SYSTEM \"http://127.0.0.1:80/ps/dtd/request.dtd\"><requestcredit username="\"abc\"" password="\"abc\""></requestcredit>&action=test";

        objRequest.Method = "POST";
        ASCIIEncoding objEncoding = new ASCIIEncoding();
        byte[] objBytes = objEncoding.GetBytes(xmlstr);
        objRequest.Timeout = 6000;
        objRequest.ContentType = "application/x-www-form-urlencoded";
        objRequest.Headers.Add("Accept-Encoding", "gzip, deflate");
        objRequest.ContentLength = objBytes.Length;
        objRequest.KeepAlive = true;

        //objRequest.ServicePoint.ConnectionLimit = GlobalVariable.connectionLimit;
        Stream strm = objRequest.GetRequestStream();
        strm.Write(objBytes, 0, objBytes.Length);
        strm.Flush();
        strm.Close();

        //ServicePointManager.DefaultConnectionLimit = GlobalVariable.connectionLimit;
        using (HttpWebResponse WebResponse = (HttpWebResponse)objRequest.GetResponse())
        {
            Stream responseStream = default(Stream);
            responseStream = WebResponse.GetResponseStream();
            if ((WebResponse.ContentEncoding.ToLower().Contains("gzip")))
            {
                responseStream = new System.IO.Compression.GZipStream(responseStream, System.IO.Compression.CompressionMode.Decompress);
            }
            else if ((WebResponse.ContentEncoding.ToLower().Contains("deflate")))
            {
                responseStream = new System.IO.Compression.DeflateStream(responseStream, System.IO.Compression.CompressionMode.Decompress);
            }

            StreamReader reader = new StreamReader(responseStream, System.Text.Encoding.Default);
            Response.Output.Write(reader.ReadToEnd());
            reader.Close();
            WebResponse.Close();
        }
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {

    } objRequest.Abort();
}