工具类 

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;
            byte[] bytes = MergeContent();
 
            try
            {
                responseBytes = webClient.UploadData(requestUrl, bytes);
                responseText = System.Text.Encoding.UTF8.GetString(responseBytes);
                return true;
            }
            catch (WebException ex)
            {
                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);
        }
    }

用法 

 private void button1_Click(object sender, EventArgs e)
        {
            var httpUpload = new HttpUpload();
            httpUpload.SetFieldValue("orderSn", "20170908");
            httpUpload.SetFieldValue("sign", "11111");
            String path = @"c:\x1.png";
            FileStream fspdf = new FileStream(path, FileMode.Open);
            byte[] fileBytepdf = new byte[fspdf.Length];
            fspdf.Read(fileBytepdf, 0, fileBytepdf.Length);
            fspdf.Close();
            var pdfName = path.Substring(path.LastIndexOf("\\") + 1);
            httpUpload.SetFieldValue("f1", pdfName, "application/octet-stream", fileBytepdf);
            string responStr = "";
            bool suc = httpUpload.Upload("http://localhost:34215/WebForm1.aspx", out responStr);
        }

接受 

 protected void Page_Load(object sender, EventArgs e)
        {
            Request.Files["f1"].SaveAs("c:\\X2.png");

            String name=    Request.Form["sign"].ToString();
                Console.WriteLine(name);
           
        }