抛出异常：

案例调用腾讯OCR营业执照识别报错。

调用腾讯OCR识别时签名没有问题，图片base64也没有问题，但是确收到对方服务报“内部错误”，经过联系沟通，内部错误报：
“caught error Error: Unexpected end of multipart data”

抓包

通过抓包如下：

POST https://recognition.image.myqcloud.com/ocr/bizlicense HTTP/1.1
Content-Type: multipart/form-data; boundary=----------8d7055498b35e31
Authorization: yzo1HrKzdUF2rmYpiRoF482vvglhPTEyNTE3OTMwODAmYj10ZW5jZW50eXVuJms9QUtJRGExRDhtQmdBbEVWWWFSemp6emg5S0l6UWl2eTJDZUJUJmU9MTU2NTMzOTcxOSZ0PTE1NjI3NDc3MTkmcj0xOTA3MTAxNjM1JnU9MCZmPQ==
Host: recognition.image.myqcloud.com
Content-Length: 249
Expect: 100-continue


------------8d7055498b35e31
Content-Disposition: form-data; name="appid";

1251793080
------------8d7055498b35e31
Content-Disposition: form-data; name="image"; filename="yyzz.jpg"
Content-Type: image/jpeg


----------8d7055498b35e31--

解决问题

乍看上面抓包好像没有什么问题，难道是Authorization加密出错？图片base64出错？

其实真正出错的的原因是Content-Type里面的横线要比body里面的少两个。body的最后一定要有两个横线结尾，并且body的横线多少要一致（这个也是最重要的，不是有横线就可以了）。

能正确运行的包应该如下：

POST https://recognition.image.myqcloud.com/ocr/bizlicense HTTP/1.1
Content-Type: multipart/form-data; boundary=----------8d7055498b35e31
Authorization: yzo1HrKzdUF2rmYpiRoF482vvglhPTEyNTE3OTMwODAmYj10ZW5jZW50eXVuJms9QUtJRGExRDhtQmdBbEVWWWFSemp6emg5S0l6UWl2eTJDZUJUJmU9MTU2NTMzOTcxOSZ0PTE1NjI3NDc3MTkmcj0xOTA3MTAxNjM1JnU9MCZmPQ==
Host: recognition.image.myqcloud.com
Content-Length: 249
Expect: 100-continue


------------8d7055498b35e31
Content-Disposition: form-data; name="appid";

1251793080
------------8d7055498b35e31
Content-Disposition: form-data; name="image"; filename="yyzz.jpg"
Content-Type: image/jpeg


------------8d7055498b35e31--

附C#调用腾讯OCR营业执照识别的部分源码

        static void IdentityBusinessLicense(string url)
        {
            HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(new Uri(url));
            Stream memStream = new MemoryStream();
            webReq.Method = "POST";
            string boundary = "----------" + DateTime.Now.Ticks.ToString("x");// 边界符  
            webReq.ContentType = "multipart/form-data; boundary=" + boundary;
            //webReq.ContentType = "multipart/form-data;";
            byte[] enter = Encoding.ASCII.GetBytes("
");  //换行
            memStream.Write(enter, 0, enter.Length);
            Dictionary<string, string> dic = new Dictionary<string, string>()
            {
                {"appid",OCRConfig.appid}
            };

            //写入文本字段
            string inputPartHeaderFormat = "--" + boundary + "
" + "Content-Disposition: form-data; name="{0}";" + "

{1}
";
            foreach (var kv in dic)
            {
                string inputPartHeader = string.Format(inputPartHeaderFormat, kv.Key, kv.Value);
                var inputPartHeaderBytes = Encoding.ASCII.GetBytes(inputPartHeader);
                memStream.Write(inputPartHeaderBytes, 0, inputPartHeaderBytes.Length);
            }
            var fileStream = new FileStream("yyzz.jpg", FileMode.Open, FileAccess.Read);
            // 写入文件  
            string imagePartHeader = "--" + boundary + "
" +
                                     "Content-Disposition: form-data; name="{0}"; filename="{1}"
" +
                                     "Content-Type: image/jpeg

";
            var header = string.Format(imagePartHeader, "image", "yyzz.jpg");
            var headerbytes = Encoding.UTF8.GetBytes(header);
            memStream.Write(headerbytes, 0, headerbytes.Length);
            var buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
            {
                memStream.Write(buffer, 0, bytesRead);
            }
            // 最后的结束符  
            byte[] endBoundary = Encoding.ASCII.GetBytes("
--" + boundary + "--
");
            memStream.Write(endBoundary, 0, endBoundary.Length);
            // webReq.ContentLength = memStream.Length;
            webReq.Headers.Add(HttpRequestHeader.Authorization, OCRApi.HmacSha1Sign());
            webReq.Host = OCRConfig.Host;
            var requestStream = webReq.GetRequestStream();
            memStream.Position = 0;
            memStream.CopyTo(requestStream);
            HttpWebResponse response = (HttpWebResponse)webReq.GetResponse();
            StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
            var ret = sr.ReadToEnd();
            sr.Close();
            response.Close();
            requestStream.Close();
            memStream.Close();
        }

总结

1、Content-Type: multipart/form-data; boundary=----------8d7055498b35e31 这里header的boundary横线要比body的少两个
2、------------8d7055498b35e31 body里面的横线要比header的横线要多两个且所有body的横线长度要一致，不是有横线就可以了。
3、特别注意结尾处应该还有两个“--”，注意是两个横线
4、还有一点就是要注意header之间，header与body之间，body与body之间的字段相隔多少个“ ”，这样就可以了。