c# post文件

public class HttpUpload
    {
        private ArrayList bytesArray;
        private Encoding encoding = Encoding.UTF8;
        private string boundary = String.Empty;

        public HttpUpload()
        {
            bytesArray = new ArrayList();
            string flag = DateTime.Now.Ticks.ToString("x");
            boundary = "---------------------------" + flag;
        }

        /// <summary>
        /// 合并请求数据
        /// </summary>
        /// <returns></returns>
        private byte[] MergeContent()
        {
            int length = 0;
            int readLength = 0;
            string endBoundary = "--" + boundary + "--\r\n";
            byte[] endBoundaryBytes = encoding.GetBytes(endBoundary);

            bytesArray.Add(endBoundaryBytes);

            foreach (byte[] b in bytesArray)
            {
                length += b.Length;
            }

            byte[] bytes = new byte[length];

            foreach (byte[] b in bytesArray)
            {
                b.CopyTo(bytes, readLength);
                readLength += b.Length;
            }

            return bytes;
        }

        /// <summary>
        /// 上传
        /// </summary>
        /// <param name="requestUrl">请求url</param>
        /// <param name="responseText">响应</param>
        /// <returns></returns>
        public bool Upload(String requestUrl, out String responseText)
        {
            WebClient webClient = new WebClient();
            webClient.Headers.Add("Content-Type", "multipart/form-data; boundary=" + boundary);

            byte[] responseBytes = null;
            byte[] bytes = MergeContent();

            try
            {
                responseBytes = webClient.UploadData(requestUrl, bytes);
                responseText = System.Text.Encoding.UTF8.GetString(responseBytes);
                return true;
            }
            catch (WebException ex)
            {
                responseText = ex.Message;
                Loger.WriteError("Update Failed. \n" + ex.Message);
                //Stream responseStream = ex.Response.GetResponseStream();
                //responseBytes = new byte[ex.Response.ContentLength];
                //responseStream.Read(responseBytes, 0, responseBytes.Length);
            }
            //responseText = System.Text.Encoding.UTF8.GetString(responseBytes);
            return false;
        }

        /// <summary>
        /// 设置表单数据字段
        /// </summary>
        /// <param name="fieldName">字段名</param>
        /// <param name="fieldValue">字段值</param>
        /// <returns></returns>
        public void SetFieldValue(String fieldName, String fieldValue)
        {
            string httpRow = "--" + boundary + "\r\nContent-Disposition: form-data; name=\"{0}\"\r\n\r\n{1}\r\n";
            string httpRowData = String.Format(httpRow, fieldName, fieldValue);

            bytesArray.Add(encoding.GetBytes(httpRowData));
        }

        /// <summary>
        /// 设置表单文件数据
        /// </summary>
        /// <param name="fieldName">字段名</param>
        /// <param name="filename">字段值</param>
        /// <param name="contentType">内容内型</param>
        /// <param name="fileBytes">文件字节流</param>
        /// <returns></returns>
        public void SetFieldValue(String fieldName, String filename, String contentType, Byte[] fileBytes)
        {
            string end = "\r\n";
            string httpRow = "--" + boundary + "\r\nContent-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n";
            string httpRowData = String.Format(httpRow, fieldName, filename, contentType);

            byte[] headerBytes = encoding.GetBytes(httpRowData);
            byte[] endBytes = encoding.GetBytes(end);
            byte[] fileDataBytes = new byte[headerBytes.Length + fileBytes.Length + endBytes.Length];

            headerBytes.CopyTo(fileDataBytes, 0);
            fileBytes.CopyTo(fileDataBytes, headerBytes.Length);
            endBytes.CopyTo(fileDataBytes, headerBytes.Length + fileBytes.Length);

            bytesArray.Add(fileDataBytes);
        }
    }

转自：https://www.cnblogs.com/password1/p/5870725.html 感谢原作者