private static string  HttpPost()
        {
            string postData="username=中文&userid=838 jdkkd";//在发送的内容里包括中文和空格
            string Web=@"http://www.myweb.com/myresponse.asp";//地址
            postData=postData.Replace(" ","%20");//替换空格
            try
            {
                //获取URL并进行发送字串
                HttpWebRequest myHttpWebRequest =(HttpWebRequest)WebRequest.Create(Web);
               
                Stream myStream=new MemoryStream ();//定义这个Stream是只是为了得到发送字串 经过编码之后得到的

byte的长度。
                StreamWriter myStreamWriter=new StreamWriter(myStream,Encoding.Default);//用默认编码 得到

Stream               
                myStreamWriter.Write(postData);
                myStreamWriter.Flush();

                long len=myStream.Length;//目的完成
                myStreamWriter.Close();
               
                myHttpWebRequest.ContentType="application/x-www-form-urlencoded";
                myHttpWebRequest.ContentLength=len;//如果字符串中存在中文 使用postData.Length得到长度和编码之

//后的长度是不一样的:(
                myHttpWebRequest.Method="POST";

                Stream newStream=myHttpWebRequest.GetRequestStream();
                myStreamWriter=new StreamWriter(newStream,Encoding.Default);//编码使用Encoding.Default 换来换去编码方法 还是用它得到的中文不会出现乱码,特别是把request("")得到的值写入sql数据库的时候 真是奇怪^O^

                myStreamWriter.Write(postData);
                myStreamWriter.Close();

                //获取本次的回馈,先创建接收回馈的类HTTPWEBRESPONSE
                HttpWebResponse myHttpWebResponse = (HttpWebResponse)myHttpWebRequest.GetResponse();
                //创建接收回馈的字节流类
                Stream receiveStream = myHttpWebResponse.GetResponseStream();//得到回写的字节流 做爬虫的基础？？？
                Encoding encode = System.Text.Encoding.Default;
                StreamReader readStream = new StreamReader( receiveStream, encode );
                readStream.Close();
                return "";
                           
            }
            catch(Exception e)
            {
                return "";
            }