C#采用HttpWebRequest实现保持会话上传文件到HTTP的方法

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。