HttpWebRequest Codes and Usage

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.NET4

i got a small piece of problem hope you can help me out.

basically. i know how to do the login in different sites using httpwebrequest..

but now, i was working with something like. i am trying to comment to one of my wordpress website,

Dim postData As String = "author=" & txtUser.Text & "&email=" & txtEmail.Text & "&url=" & txtWebsite.Text & "&comment=" & txtMessage.Text & "&submit=&comment_post_ID=" & stringA
        Dim tempcookies As New CookieContainer
        Dim encoding As New UTF8Encoding
        Dim byteData As Byte() = encoding.GetBytes(postData)
        Dim postreq As HttpWebRequest = DirectCast(HttpWebRequest.Create("http://www.myurl.com/wp-comments-post.php"), HttpWebRequest)
        postreq.Method = "POST"
  
        postreq.KeepAlive = True
        postreq.CookieContainer = tempcookies
        postreq.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 6.1; ru; rv:1.9.2.3) Gecko/20100401 Firefox/4.0 (.NET CLR 3.5.30729"
        postreq.ContentType = "application/x-www-form-urlencoded"
        postreq.Referer = "url"
        postreq.ContentLength = byteData.Length
        Dim postreqstream As Stream = postreq.GetRequestStream()
        postreqstream.Write(byteData, 0, byteData.Length)
        postreqstream.Close()
        Dim postresponse As HttpWebResponse
        postresponse = DirectCast(postreq.GetResponse, HttpWebResponse)
        tempcookies.Add(postresponse.Cookies)

but i am stuck with the post respone i'm getting an error of.
The remote server returned an error: (500) Internal Server Error.

but when I will try to login in wordpress, with the format of login post data.. it logs me,

This is the one you can see on LiveHttpHeaders

'author=a57rf&email=abcdefg%40yahoo.com&url=http%3A%2F%2Fyah.com&comment=xxxxxx+sample&submit=&comment_post_ID=8

 
'if I substitute the values to my variables like below, it gives me an error 500, Internal Server Error

Dim postData As String = "author=" & txtUser.Text & "&email=" & txtEmail.Text & "&url=" & txtWebsite.Text & "&comment=" & txtMessage.Text & "&submit=&comment_post_ID=" & stringA

tried to remove the &comment_post_ID , it woudn't give me an error, but it doesnt give me a result also, its like a blank result.


the referURL, is originally a url, i just substitute it temporarily as "url", but its really a url inside

do you have an idea how to tweak this?