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; }