这边我以big5转换gb2312为例
Encoding big5 =Encoding.GetEncoding("big5");
Encoding gb2312 = Encoding.GetEncoding("gb2312");
byte[] big5b= big5.GetBytes("編程無悔!");
//关键也就是这句了
byte[] gb2312b= Encoding.Convert(big5,gb2312,big5b);
string strGb2312 = gb2312.GetString(gb2312b)