无法判断目标网站编码的解决方法

/// <summary>
/// 函数名称：GetDataFromUrl
/// 功能说明：获取url指定的网页的源码
/// 参数：string url用于指定 url
/// 参数：ref Encoding encode用来获取网页中的字符集编码
/// </summary>
public static string GetDataFromUrl(string url, ref Encoding encode)
{
string str = string.Empty;
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);

//设置http头
request.AllowAutoRedirect = true;
request.AllowWriteStreamBuffering = true;
request.Referer = "";
request.Timeout = 10 * 1000;
request.UserAgent = "";
HttpWebResponse response = null;
response = (HttpWebResponse)request.GetResponse();

//根据http应答的http头来判断编码
string characterSet = response.CharacterSet;
//Encoding encode;
if (characterSet != "")
{
if (characterSet == "ISO-8859-1")
{
characterSet = "gb2312";
}
encode = Encoding.GetEncoding(characterSet);
}
else
{
encode = Encoding.Default;
}

//声明一个内存流来保存http应答流
Stream receiveStream = response.GetResponseStream();
MemoryStream mStream = new MemoryStream();

byte[] bf = new byte[255];
int count = receiveStream.Read(bf, 0, 255);
while (count > 0)
{
mStream.Write(bf, 0, count);
count = receiveStream.Read(bf, 0, 255);
}
receiveStream.Close();

mStream.Seek(0, SeekOrigin.Begin);

//从内存流里读取字符串
StreamReader reader = new StreamReader(mStream, encode);
char[] buffer = new char[1024];
count = reader.Read(buffer, 0, 1024);
while (count > 0)
{
str += new String(buffer, 0, count);
count = reader.Read(buffer, 0, 1024);
}

//从解析出的字符串里判断charset，如果和http应答的编码不一直
//那么以页面声明的为准，再次从内存流里重新读取文本
Regex reg =
new Regex(@"<meta[\s\S]+?charset=(.*?)""[\s\S]+?>",
RegexOptions.Multiline | RegexOptions.IgnoreCase);
MatchCollection mc = reg.Matches(str);
if (mc.Count > 0)
{
string tempCharSet = mc[0].Result("$1");
if (string.Compare(tempCharSet, characterSet, true) != 0)
{
encode = Encoding.GetEncoding(tempCharSet);
str = string.Empty;
mStream.Seek(0, SeekOrigin.Begin);
reader = new StreamReader(mStream, encode);
buffer = new char[255];
count = reader.Read(buffer, 0, 255);
while (count > 0)
{
str += new String(buffer, 0, count);
count = reader.Read(buffer, 0, 255);
}
}
}
reader.Close();
mStream.Close();
if (response != null)
response.Close();

return str;

}