调用webservice的soapheader问题

Question

public void test()
{
StringBuilder soap = new StringBuilder();
            soap.Append("<?xml version=\"1.0\" encoding=\"utf-8\"?>");
            soap.Append("<soap:Envelope xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\">");
            soap.Append("<soap:Header>");
            soap.Append("<MySoapHeader xmlns=\"http://www.test.com/\" soap:mustUnderstand=\"1\">");
            soap.Append("<Username>admin</Username>");
            soap.Append("<Password>admin123</Password>");
            soap.Append("</MySoapHeader>");
            soap.Append("</soap:Header>");
            soap.Append("<soap:Body>");
            soap.Append("<getSecuritiesTreeByRemote xmlns=\"http://www.test.com/\">");
            soap.Append("</getSecuritiesTreeByRemote>");
            soap.Append("</soap:Body>");
            soap.Append("</soap:Envelope>");
            //Uri uri = new Uri("http://www.webxml.com.cn/WebServices/WeatherWS.asmx");
            Uri uri = new Uri(strURL);
            WebRequest webRequest = WebRequest.Create(uri);
            webRequest.ContentType = "text/xml; charset=utf-8";
            webRequest.Method = "POST";
            using (Stream requestStream = webRequest.GetRequestStream())
            {
                byte[] paramBytes = Encoding.UTF8.GetBytes(soap.ToString());
                requestStream.Write(paramBytes, 0, paramBytes.Length);
            }
            
            //响应
            WebResponse webResponse = webRequest.GetResponse();
            using (StreamReader myStreamReader = new StreamReader(webResponse.GetResponseStream(), Encoding.UTF8))
            {
                Console.WriteLine(myStreamReader.ReadToEnd());
            }            Console.ReadKey()
}
我用了上面的代码调用webservice，但现在服务器端加了验证，需要在soapheader里增加用户名和密码的信息才能访问到方法，我试了如上代码，但报500错误，不知为什么：（