问System.Net.HttpWebRequest.GetResponse()错误“您必须在调用[Begin]GetResponse之前将ContentLength字节写入请求流。”

EN

<httpRuntime maxRequestLength="3548000" executionTimeout="1000" targetFramework="4.6.1"/>

        HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(uri);
        webrequest.CookieContainer = cookies;
        webrequest.ContentType = "multipart/form-data; boundary=" + boundary;
        webrequest.Method = "POST";

        // Build up the post message header
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("--"); sb.Append(boundary); sb.Append("\r\n");
        sb.Append("Content-Disposition: form-data; name=\""); sb.Append(fileFormName); 
        sb.Append("\"; filename=\""); sb.Append(Path.GetFileName(uploadfile)); sb.Append("\""); sb.Append("\r\n");
        sb.Append("Content-Type: "); sb.Append("application/octet-stream"); sb.Append("\r\n");
        sb.Append("\r\n");            

        string postHeader = sb.ToString();
        byte[] postHeaderBytes = System.Text.Encoding.UTF8.GetBytes(postHeader);
        // Build the trailing boundary string as a byte array. ensuring the boundary appears on a line by itself
        byte[] boundaryBytes =  System.Text.Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");

        string strRead = "";
        FileStream fileStream = null;
        Stream requestStream = null;
        WebResponse responce = null;
        StreamReader responseStream = null;

        try
        {
            fileStream = new FileStream(uploadfile, FileMode.Open, FileAccess.Read);
            long length = postHeaderBytes.Length + fileStream.Length + boundaryBytes.Length;
            webrequest.ContentLength = length;

            using (requestStream = webrequest.GetRequestStream())
            {
                //requestStream = webrequest.GetRequestStream();
                // Write out our post header
                requestStream.Write(postHeaderBytes, 0, postHeaderBytes.Length);
                // Write out the file contents
                byte[] buffer = new Byte[checked((uint)Math.Min(4096, (int)fileStream.Length))];
                int bytesRead = 0;
                while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0) { requestStream.Write(buffer, 0, bytesRead); }
                // Write out the trailing boundary
                requestStream.Write(boundaryBytes, 0, boundaryBytes.Length);

                responce = webrequest.GetResponse();
            }
            responseStream = new StreamReader(responce.GetResponseStream());

            strRead = responseStream.ReadToEnd();
        }
        catch (Exception ex)
        {
                     // handle error
        }
        finally
        {
            if( fileStream != null ) fileStream.Close();
            if( requestStream != null ) requestStream.Close();
            if( responseStream != null ) responseStream.Close();
            if( responce != null ) responce.Close();
        }
        return strRead;