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：
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。