string data = string.Empty;
            System.Drawing.Bitmap bmp1 = new System.Drawing.Bitmap(@"C:\Users\yafa2\Desktop\11.jpg");
            using (MemoryStream ms1 = new MemoryStream())
            {
                bmp1.Save(ms1, System.Drawing.Imaging.ImageFormat.Jpeg);
                byte[] arr1 = new byte[ms1.Length];
                ms1.Position = 0;
                ms1.Read(arr1, 0, (int)ms1.Length);
                ms1.Close();
                data = Convert.ToBase64String(arr1);
            }
            Encoding encoding = System.Text.Encoding.Default;
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://192.9.100.36:5012/UpDownFile.asmx/UpImgByApp");
           
            System.Text.StringBuilder builder = new System.Text.StringBuilder(); 
            builder.AppendFormat("{0}={1}&", "data",data);//此处是图片的64位字符编码，但是会超长，请问怎么解决，
            builder.AppendFormat("{0}={1}&", "fileName", "110000200069_201908151740218.jpg");
            builder.AppendFormat("{0}={1}", "id", "110000200069_201908151740281");

            byte[] postData = System.Text.Encoding.UTF8.GetBytes(builder.ToString());

            req.Method = "POST";
            req.ContentType = "application/x-www-form-urlencoded";
            req.Timeout = 1000 * 30;
            req.ContentLength = postData.Length;

            using (System.IO.Stream reqStream = req.GetRequestStream())
            {
                reqStream.Write(postData, 0, postData.Length);
                reqStream.Close();
            }
            System.Net.HttpWebResponse resp = (System.Net.HttpWebResponse)req.GetResponse();
            System.IO.Stream stream = resp.GetResponseStream();
            //获取响应内容
            using (System.IO.StreamReader reader = new System.IO.StreamReader(stream, System.Text.Encoding.UTF8))
            {
                data = reader.ReadToEnd();
            }
            MessageBox.Show(data);

在post man 中没问题，为什么在c#中 就是那个data超长呢？？postman用的是body方式提的，怎么用c#body提交？？ 我试过了form data方式不行