页面Post提交后，提交的数据变乱码的问题，请大家帮帮我！

protected static string cookieheader;
public string Login(String url, String paramList) 
{
HttpWebResponse res = null;
string strResult=""; try 
{
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
req.AllowAutoRedirect = false;
CookieContainer cookieCon = new CookieContainer();
req.CookieContainer = cookieCon; StringBuilder UrlEncoded = new StringBuilder();
Char[] reserved = {'?', '=', '&'};
byte[] SomeBytes = null; if (paramList != null) 
{
int i=0, j;
while(i<paramList.Length)
{
j=paramList.IndexOfAny(reserved, i);
if (j==-1)
{
UrlEncoded.Append(HttpUtility.UrlEncode(paramList.Substring(i, paramList.Length-i)));
break;
}
UrlEncoded.Append(HttpUtility.UrlEncode(paramList.Substring(i, j-i)));
UrlEncoded.Append(paramList.Substring(j,1));
i = j+1;
}
SomeBytes = Encoding.UTF8.GetBytes(UrlEncoded.ToString());
req.ContentLength = SomeBytes.Length;
Stream newStream = req.GetRequestStream();
newStream.Write(SomeBytes, 0, SomeBytes.Length);
newStream.Close();
} 
else 
{
req.ContentLength = 0;
} res = (HttpWebResponse)req.GetResponse();
cookieheader = req.CookieContainer.GetCookieHeader(new Uri(url));
HttpContext.Current.Application.Lock();
HttpContext.Current.Application["cookieheader"] = cookieheader;
HttpContext.Current.Application.UnLock(); Stream ReceiveStream = res.GetResponseStream();
Encoding encode = System.Text.Encoding.UTF8;
StreamReader sr = new StreamReader( ReceiveStream, encode );
Char[] read = new Char[256];
int count = sr.Read( read, 0, 256 );
while (count > 0) 
{
String str = new String(read, 0, count);
strResult += str;
count = sr.Read(read, 0, 256);
}
} 
catch(Exception e) 
{
strResult = e.ToString();
} 
finally 
{
if ( res != null ) 
{
res.Close();
}
}
strResult = strResult.Replace("\r\n","");
return strResult;
}上面的是post提交函数，已经无数次测试了提交能够成功，可是当如果参数是中文时，提交过去的编码按utf_8编码了，而提交过去的网页是按gb2312编码的（编码不可更改）,我怎么样把我参数里面的中文用gb2312进行编码后再提交，有这方面经验的兄弟，请指导一下小弟，我已经把里面的编码改了无数遍了，都没用。。