写入后ContentLength未设置

Question

我正在尝试使用此功能将数据发送到服务器：

public static async Task<Byte[]> sendData(Byte[] bt)
    {
        try
        {
            System.Net.HttpWebRequest myWebRequest = (HttpWebRequest)System.Net.HttpWebRequest.Create(URL);
            myWebRequest.Method = "POST";
            myWebRequest.ContentType = "text/xml";
            myWebRequest.AllowReadStreamBuffering = true;
            using (var requestStream = (Stream)(await Task<Stream>.Factory.FromAsync(myWebRequest.BeginGetRequestStream, myWebRequest.EndGetRequestStream, myWebRequest)))
            {
                requestStream.Write(bt, 0, bt.Length);
            }

            using (var response = (HttpWebResponse)(await Task<WebResponse>.Factory.FromAsync(myWebRequest.BeginGetResponse, myWebRequest.EndGetResponse, myWebRequest)))
            {
                using (var responseStream = response.GetResponseStream())
                {
                    bt = new byte[responseStream.Length];
                    responseStream.Read(bt, 0, bt.Length);
                    return bt;
                }
            }
        }
        catch (Exception ex)
        {
            return null;
        }

    }

我收到服务器的回复，说没有发送数据（ContentLength == 0）。实际上，如果我在流请求中写入后查看myWebRequest.ContentLength，则_ContentLength设置为0，ContentLength设置为-1。

我错过了什么吗？

感谢您的帮助！