Friday, May 29, 2009

WebClient Object

Long time since the last post...

I've been working on a project that interfaces with numerous HTTP servers to update the firmware on the hosting machine. I opted to use the WebClient object, mostly because I wasn't sure what the best method would be but figured using WebBrowsers would just be wasteful, and ran into a few problems that were tedious enough to make me want to post them on here.

First thing's first, some servers respond back with an "Expect" error, equating to an exception from the WebClient. The fix:

System.Net.ServicePointManager.Expect100Continue = false;

Secondly, logging into a website. The WebClient object doesn't handle cookies for you and that sucks, a lot. The easiest way to fix that is by creating a new class, inheriting the WebClient class and overriding the GetWebRequest method.

class WebClientEx : WebClient
private CookieContainer m_container = new CookieContainer();

protected override WebRequest GetWebRequest(Uri address)
WebRequest request = base.GetWebRequest(address);
if (request is HttpWebRequest)
(request as HttpWebRequest).CookieContainer = m_container;
return request;
Credit for this goes to

Ok, that gets you cookies but submitting to a form wasn't all that easy...

// Create the POST form parameters
NameValueCollection nvc = new NameValueCollection();
nvc.Add("authUser", username);
nvc.Add("authPass", password);

// Pause until m_client is available
while (m_client.IsBusy)

result = System.Text.ASCIIEncoding.ASCII.GetString(
m_client.UploadValues("http://...", "POST", nvc));

And finally, the last thing I had to figure out how to do was upload a file, via a form. Fortunately, I was able to find a piece of code from madmik3 that did the trick.

// Majority of this function from
public string FormUploadFile(string uploadfile, string url, string fileFormName, string contenttype, NameValueCollection querystring)
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 = this.GetWebRequest(uri) as HttpWebRequest; // Changed by Austin 5/22/09 - To account for cookies
webrequest.ContentType = "multipart/form-data; boundary=" + boundary;
webrequest.Method = "POST";

// Build up the post message header
StringBuilder sb = new StringBuilder();
sb.Append("Content-Disposition: form-data; name=\"");
sb.Append("\"; filename=\"");
sb.Append("Content-Type: ");

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("\r\n--" + boundary + "\r\n");

FileStream fileStream = new FileStream(uploadfile,
FileMode.Open, FileAccess.Read);
long length = postHeaderBytes.Length + fileStream.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 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();

1 comment:

Sumit Jaiswal said...

Awesome work :)
Extremely Helpful
Thanks a Ton!!!!!!