asp.net 编码格式的问题

方法： public string Base64Decode(string Message)
{
try
{
System.Text.UTF7Encoding encoder = new System.Text.UTF7Encoding();
System.Text.Decoder utf8Decode = encoder.GetDecoder();
byte[] todecode_byte = Convert.FromBase64String(Message);
int charCount = utf8Decode.GetCharCount(todecode_byte, 0, todecode_byte.Length);
char[] decoded_char = new char[charCount];
utf8Decode.GetChars(todecode_byte, 0, todecode_byte.Length, decoded_char, 0);
string result = new String(decoded_char);
return result;
}
catch (Exception e)
{
throw new Exception("Error in base64Decode" + e.Message);
}
}

调用： String b = Base64Decode("TaX8YVYGhV/x+iBKugxPEA==");
b为 ：

        Response.ContentEncoding = Encoding.UTF8;
        Response.Charset = "utf-8";
        Response.Write(b);

输出到页面变成了

有些特殊字符不显示出来了，应该怎样处理了，查了很多资料一直得不到解决，求帮助