本文实例讲述了C#采用HttpWebRequest实现保持会话上传文件到HTTP的方法，在项目开发中有一定的实用价值，具体方法如下：
一、前言：
这篇文章翻译来自madmik3 写在 CodeProject 上的文章，原标题为: C#'s WebClient.UploadFile with more functionality.
二、正文：
我们使用 WebRequest 来获取网页内容是非常简单的，可是用他来上传文件就没有那么简单了。
如果我们在网页中上传文件，加入下面代码即可：
HTML 文件上传代码实例：
<form action ="http://localhost/test.php" method = POST> <input type = text name = uname> <input type = password name =passwd> <input type = FILE name = uploadfile> <input type=submit> </form>
但，如果在C#中使用 WebRequest 上传，必须对本地文件进行相应的处理才能提交到指定的HTTP地址，下面这个函数哦就帮我们做了这烦恼的操作
UploadFileEx 上传文件函数：
public static string UploadFileEx( string uploadfile, string url, string fileFormName, string contenttype,NameValueCollection querystring, CookieContainer cookies) { if( (fileFormName== null) || (fileFormName.Length ==0)) { fileFormName = "file"; } if( (contenttype== null) || (contenttype.Length ==0)) { contenttype = "application/octet-stream"; } string postdata; postdata = ""; if (querystring!=null) { foreach(string key in querystring.Keys) { postdata+= key +"=" + querystring.Get(key)+"&"; } } Uri uri = new Uri(url+postdata); string boundary = "----------" + DateTime.Now.Ticks.ToString("x"); HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(uri); webrequest.CookieContainer = cookies; webrequest.ContentType = "multipart/form-data; boundary=" + boundary; webrequest.Method = "POST"; // Build up the post message header StringBuilder sb = new StringBuilder(); sb.Append("--"); sb.Append(boundary); sb.Append(""); sb.Append("Content-Disposition: form-data; name=""); sb.Append(fileFormName); sb.Append(""; filename=""); sb.Append(Path.GetFileName(uploadfile)); sb.Append("""); sb.Append(""); sb.Append("Content-Type: "); sb.Append(contenttype); sb.Append(""); sb.Append(""); string postHeader = sb.ToString(); byte[] postHeaderBytes = Encoding.UTF8.GetBytes(postHeader); // Build the trailing boundary string as a byte array // ensuring the boundary appears on a line by itself byte[] boundaryBytes = Encoding.ASCII.GetBytes("--" + boundary + ""); FileStream fileStream = new FileStream(uploadfile, FileMode.Open, FileAccess.Read); long length = postHeaderBytes.Length + fileStream.Length + boundaryBytes.Length; webrequest.ContentLength = length; Stream requestStream = webrequest.GetRequestStream(); // Write out our post header requestStream.Write(postHeaderBytes, 0, postHeaderBytes.Length); // Write out the file contents byte[] buffer = new Byte[checked((uint)Math.Min(4096, (int)fileStream.Length))]; int bytesRead = 0; while ( (bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0 ) requestStream.Write(buffer, 0, bytesRead); // Write out the trailing boundary requestStream.Write(boundaryBytes, 0, boundaryBytes.Length); WebResponse responce = webrequest.GetResponse(); Stream s = responce.GetResponseStream(); StreamReader sr = new StreamReader(s); return sr.ReadToEnd(); }
调用代码如下：
CookieContainer cookies = new CookieContainer(); //add or use cookies NameValueCollection querystring = new NameValueCollection(); querystring["uname"]="uname"; querystring["passwd"]="snake3"; string uploadfile;// set to file to upload uploadfile = "c:test.jpg"; //everything except upload file and url can be left blank if needed string outdata = UploadFileEx(uploadfile, "http://localhost/test.php","uploadfile", "image/pjpeg", querystring,cookies);
至此，所有主要功能代码都已经介绍完毕，至于美化你的程序就要读者根据自己的喜好完成了。
作者还提供了接收文件的PHP代码，，个人觉得既然能写上传的程序，接收文件的Server Page应该不在话下的，况且又不是PHP才能接收提交的文件，ASP，ASP.NET,JSP都可以，这里就不再啰嗦了。
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