Encoding.GetEncoding("GBK").GetString(Encoding.Default.GetBytes(str))

------解决方案--------------------

string   s="";  
byte[]   buffer= Encoding.GetEncoding( "GBK "); .GetBytes(S);  
byte[]   bUtf8=Encoding.Convert(Encoding.GetEncoding( "GBK"),Encoding.UTF8,   buffer);  
Text =Encoding.UTF8.GetString(bUtf8);