The remote server returned an error: (500) Internal Server Error. how to solve this problem

i used this coding in asp.net web application. may be the requestinng arguement(userid and password) is not correct format. i think this blow coding for consol application. how to send those arguement in asp.net web application ? or what is the problem?

public void functionname()
{

HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create("http://xxxxx/function.asmx/login");

myRequest.Method = "POST";

using (StreamWriter requestWriter = new StreamWriter(myRequest.GetRequestStream()))
{

requestWriter.Write("&{0}={1}", HttpUtility.UrlEncode("UserId"), HttpUtility.UrlEncode("christ"));

requestWriter.Write("&{0}={1}", HttpUtility.UrlEncode("Password"), HttpUtility.UrlEncode("christ123"));


}

using (StreamReader responseReader = new StreamReader(myRequest.GetResponse().GetResponseStream())) // here throws internal error. it is not going inside this function.
{

result = responseReader.ReadToEnd();

}

}