public string GB2312ToUtf8(string gb2312String)
{
    Encoding fromEncoding = Encoding.GetEncoding("gb2312");
    Encoding toEncoding = Encoding.UTF8;
return EncodingConvert(gb2312String, fromEncoding, toEncoding);
}

public string Utf8ToGB2312(string utf8String)
{
    Encoding fromEncoding = Encoding.UTF8;
    Encoding toEncoding = Encoding.GetEncoding("gb2312");
return EncodingConvert(utf8String, fromEncoding, toEncoding);
}

public string EncodingConvert(string fromString, Encoding fromEncoding, Encoding toEncoding)
{            
byte[] fromBytes = fromEncoding.GetBytes(fromString);
byte[] toBytes = Encoding.Convert(fromEncoding, toEncoding, fromBytes);

string toString = toEncoding.GetString(toBytes);
return toString;
}