I'm using the Html Agility Pack for getting and parsing HTML pages. It offers many possibilities, including XPath selectors.

Today I had a problem with posting data to a webpage. It's possible to call the HtmlWeb Load function with a second "POST" parameter, but it offers no easy way to add values to be posted.
After a while of searching I came across this solution: add this two functions to the HtmlWeb class. (You can get the source in download section, too)

public HtmlDocument SubmitFormValues (NameValueCollection fv, string url)
{
        // Attach a temporary delegate to handle attaching
        // the post back data
        PreRequestHandler handler = delegate
                (HttpWebRequest request) {
                string payload = this.AssemblePostPayload (fv)
                ;
                        byte
                [
                ] buff = Encoding.ASCII.GetBytes (payload.ToCharArray (
                )
                )
                ;
                        request.ContentLength = buff.Length
                ;
                        request.ContentType = "application/x-www-form-urlencoded"
                ;
                        System.IO
                .Stream reqStream = request.GetRequestStream (
                )
                ;
                        reqStream.Write (buff, 0, buff.Length
                )
                ;
                        return true;
        }
        this.PreRequest += handler;
        HtmlDocument doc = this.Load (url, "POST"
                )
                ;
        this.PreRequest -= handler;
        return doc;
}
 
private string AssemblePostPayload (NameValueCollection fv)
{
        StringBuilder sb = new StringBuilder (
                )
                ;
        foreach (
                String key in fv.AllKeys
                ) {
                sb.Append (
                "&" + key + "=" + fv.Get (key)
                )
                ;
        }
        return sb.ToString (
                ).Substring (
                1
                )
                ;
}

Source: http://htmlagilitypack.codeplex.com/Thread/View.aspx?ThreadId=14255

Now posting is easy. Example:

NameValueCollection postData = new NameValueCollection (
                1
                )
                ;
postData.Add (
                "name", "value"
                )
                ;
doc = this.hw.SubmitFormValues (postData, url)
                ;

(hw is an HtmlWeb object and doc an HtmlDocument object)

If you need to use cookies, too, you need to modify the HtmlWeb a little more, because there is a little bug.
First add a new private variable:

private CookieContainer _ck = new CookieContainer(
                )
                ;

Then find this section:

if (_useCookies) 
{
        req.CookieContainer = new CookieContainer(
                )
                ;
}

And change it into:

if (_useCookies) 
{
        req.CookieContainer = this._ck;
}

Now you just have to activate the usage of cookies in your HtmlWeb object:

this.hw = new HtmlWeb (
                )
                ;
this.hw.UseCookies = true;

That's it! Enjoy easy form posting with a full cookie jar (: