C#发送HttpPost请求来调用WebService的方法

void updatecontactsign()
        {
           string serverpage ="http://localhost/webservice/myservice.asmx";
            try
            {
                //serverpage += "?op=tangramaction";
                serverpage += "/myaction";//myaction是webservice中的方法
           string strxml="<a objid=\"9\"></a>",;//第一个参数
           string strdata="contactsign|990011|我的数据";//第二个参数
           string res = httpconnecttoserver(serverpage, strxml, strdata);
                //messagebox.show(res);
            }
            catch (exception ex)
            {

            }
        }

        //发送消息到服务器
      public string httpconnecttoserver(string serverpage,string strxml,string strdata)
        {
            string postdata = "strxml=" + strxml+"&strdata="+strdata;

            byte[] dataarray = encoding.default.getbytes(postdata);
            //创建请求
            httpwebrequest request = (httpwebrequest)httpwebrequest.create(serverpage);
            request.method = "post";
            request.contentlength = dataarray.length;
            request.contenttype = "application/x-www-form-urlencoded";
            //创建输入流
            stream datastream = null;
            try
            {
                datastream = request.getrequeststream();
            }
            catch (exception)
            {
                return null;//连接服务器失败
            }

            //发送请求
            datastream.write(dataarray, 0, dataarray.length);
            datastream.close();
            //读取返回消息
            string res = string.empty;
            try
            {
                httpwebresponse response = (httpwebresponse)request.getresponse();
                streamreader reader = new streamreader(response.getresponsestream(), encoding.utf8);
                res = reader.readtoend();
                reader.close();
            }
            catch (exception ex)
            {
                return null;//连接服务器失败
            }
            return res;
        }