C# Body为form-data file文件上传至第三方接口

1.第三方提供了这么一个API接口，POST后将附件以file文件上传，通过postman工具界面是下面这种样子

Body为form-data类型，不管是文本还是文件，都能在value上填上去

2.C#实现接口调用代码

public static string PostForm(string url, List<FormItemModel> formItems, CookieContainer cookieContainer = null, string refererUrl = null, Encoding encoding = null, int timeOut = 20000)
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            #region 初始化请求对象
            request.Method = "POST";
            request.Timeout = timeOut;
            request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8";
            request.KeepAlive = true;
            request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.57 Safari/537.36";
            if (!string.IsNullOrEmpty(refererUrl))
                request.Referer = refererUrl;
            if (cookieContainer != null)
                request.CookieContainer = cookieContainer;
            #endregion

            string boundary = "----" + DateTime.Now.Ticks.ToString("x");//分隔符
            request.ContentType = string.Format("multipart/form-data; boundary={0}", boundary);
            //请求流
            var postStream = new MemoryStream();
            #region 处理Form表单请求内容
            //是否用Form上传文件
            var formUploadFile = formItems != null && formItems.Count > 0;
            if (formUploadFile)
            {
                //文件数据模板
                string fileFormdataTemplate =
                    "\r\n--" + boundary +
                    "\r\nContent-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"" +
                    "\r\nContent-Type: application/octet-stream" +
                    "\r\n\r\n";
                //文本数据模板
                string dataFormdataTemplate =
                    "\r\n--" + boundary +
                    "\r\nContent-Disposition: form-data; name=\"{0}\"" +
                    "\r\n\r\n{1}";
                foreach (var item in formItems)
                {
                    string formdata = null;
                    if (item.IsFile)
                    {
                        //上传文件
                        formdata = string.Format(
                            fileFormdataTemplate,
                            item.Key, //表单键
                            item.FileName);
                    }
                    else
                    {
                        //上传文本
                        formdata = string.Format(
                            dataFormdataTemplate,
                            item.Key,
                            item.Value);
                    }

                    //统一处理
                    byte[] formdataBytes = null;
                    //第一行不需要换行
                    if (postStream.Length == 0)
                        formdataBytes = Encoding.UTF8.GetBytes(formdata.Substring(2, formdata.Length - 2));
                    else
                        formdataBytes = Encoding.UTF8.GetBytes(formdata);
                    postStream.Write(formdataBytes, 0, formdataBytes.Length);

                    //写入文件内容
                    if (item.FileContent != null && item.FileContent.Length > 0)
                    {
                        using (var stream = item.FileContent)
                        {
                            byte[] buffer = new byte[1024];
                            int bytesRead = 0;
                            while ((bytesRead = stream.Read(buffer, 0, buffer.Length)) != 0)
                            {
                                postStream.Write(buffer, 0, bytesRead);
                            }
                        }
                    }
                }
                //结尾
                var footer = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n");
                postStream.Write(footer, 0, footer.Length);

            }
            else
            {
                request.ContentType = "application/x-www-form-urlencoded";
            }
            #endregion

            request.ContentLength = postStream.Length;

            #region 输入二进制流
            if (postStream != null)
            {
                postStream.Position = 0;
                //直接写入流
                Stream requestStream = request.GetRequestStream();

                byte[] buffer = new byte[1024];
                int bytesRead = 0;
                while ((bytesRead = postStream.Read(buffer, 0, buffer.Length)) != 0)
                {
                    requestStream.Write(buffer, 0, bytesRead);
                }

                ////debug
                //postStream.Seek(0, SeekOrigin.Begin);
                //StreamReader sr = new StreamReader(postStream);
                //var postStr = sr.ReadToEnd();
                postStream.Close();//关闭文件访问
            }
            #endregion

            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            if (cookieContainer != null)
            {
                response.Cookies = cookieContainer.GetCookies(response.ResponseUri);
            }

            using (Stream responseStream = response.GetResponseStream())
            {
                using (StreamReader myStreamReader = new StreamReader(responseStream, encoding ?? Encoding.UTF8))
                {
                    string retString = myStreamReader.ReadToEnd();
                    return retString;
                }
            }
        }

public class FormItemModel
    {
        /// <summary>
        /// 表单键，request["key"]
        /// </summary>
        public string Key { set; get; }
        /// <summary>
        /// 表单值,上传文件时忽略，request["key"].value
        /// </summary>
        public string Value { set; get; }
        /// <summary>
        /// 是否是文件
        /// </summary>
        public bool IsFile
        {
            get
            {
                if (FileContent == null || FileContent.Length == 0)
                    return false;

                if (FileContent != null && FileContent.Length > 0 && string.IsNullOrWhiteSpace(FileName))
                    throw new Exception("上传文件时 FileName 属性值不能为空");
                return true;
            }
        }
        /// <summary>
        /// 上传的文件名
        /// </summary>
        public string FileName { set; get; }
        /// <summary>
        /// 上传的文件内容
        /// </summary>
        public Stream FileContent { set; get; }



    }

模拟post调用

var url = "http://127.0.0.1/testformdata.aspx?aa=1&bb=2&ccc=3";
var log1=@"D:\temp\log1.txt";
var log2 = @"D:\temp\log2.txt";
var formDatas = new List<Grass.Net.FormItemModel>();
//添加文件
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key="log1",
    Value="",
    FileName = "log1.txt",
    FileContent=File.OpenRead(log1)
});
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key = "log2",
    Value = "",
    FileName = "log2.txt",
    FileContent = File.OpenRead(log2)
});
//添加文本
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key = "id",
    Value = "id-test-id-test-id-test-id-test-id-test-"
});
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key = "name",
    Value = "name-test-name-test-name-test-name-test-name-test-"
});
//提交表单
var result = PostForm(url, formDatas);

至此引用别人的资料：https://www.cnblogs.com/xiaosuxing/p/13403172.html

3.以上方法读流是通过文件存取的绝对路径去转换为流，进而去请求第三方接口把文件以流的方式上传
如果是文件服务器上的URL，则此方法不能通用，也试过将流转换都不行，最后找到一种思路可行，先将URL下载存到临时文件夹里，再通过以上方法将文件读取成流后上传，代码如下

/// <summary>
        ///  根据文件服务器上的地址URL保存文件到临时路径
        /// </summary>
        /// <param name="url"></param>
        /// <returns></returns>
        public static Stream HttpDownload(string url)
        {
            using (var client = new WebClient())
            {
                string tempFile = Path.GetTempFileName();
                client.DownloadFile(url, tempFile);//下载临时文件
                return FileToStream(tempFile, true);//转文件流后删除临时文件夹
            }
        }

        /// <summary>
        /// 文件转流
        /// </summary>
        /// <param name="fileName">文件路径</param>
        /// <param name="isDelete">是否删除临时文件</param>
        /// <returns></returns>
        public static Stream FileToStream(string fileName, bool isDelete = false)
        {

            //打开文件

            FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);

            // 读取文件的 byte[]

            byte[] bytes = new byte[fileStream.Length];

            fileStream.Read(bytes, 0, bytes.Length);

            fileStream.Close();

            // 把 byte[] 转换成 Stream

            Stream stream = new MemoryStream(bytes);
            if (isDelete)
            {
                File.Delete(fileName);//删除临时文件
            }
            return stream;

        }