Serial.println and port.readLine() in c# seem to read different strings

i have a problem with the string from serial.println then interfacing it with serialport.readline().
this from UID RC522

void loop() {
     if ( ! mfrc522.PICC_IsNewCardPresent()) {
  return;
 }
        if ( ! mfrc522.PICC_ReadCardSerial()) {
  return;
 }
String rfidUid = "";
for (byte i = 0; i < mfrc522.uid.size; i++) {
  rfidUid += String(mfrc522.uid.uidByte[i] < 0x10 ? "0" : "");
  rfidUid += String(mfrc522.uid.uidByte[i], HEX);
}
//Serial.print("UID: ");
//rfidUid.toUpperCase();
Serial.println(rfidUid);
mfrc522.PICC_HaltA();
}

and this from c#

serialPort1.Open();
string idtag = serialPort1.ReadLine();
id.Text = idtag;
serialPort1.Open();
con.Open();
            MySqlCommand cmd = con.CreateCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "select * from member where nama='" + na.Text + "' and id='" + id.Text + "'"e
            cmd.ExecuteNonQuery();
            DataTable dt = new DataTable();
            MySqlDataAdapter da = new MySqlDataAdapter(cmd);
            da.Fill(dt);
            i = Convert.ToInt32(dt.Rows.Count.ToString());

            if (i == 0)
            {
                label4.Text = "card invalid!";
                label5.Text = "try again !";
            }
            else
            {
                label4.Text = "Log in Success !";
                label5.Text = "================";
            }
            con.Close();

=====================================================

i have try to get the string as id(password) for login, but it failed even the id from serial same as the data in database. serial.println and serial.readline() seem to read different string.
can you help me or tell me why?

sorry about my english before,

dbs.PNG

form.PNG

I'm sure .net uses unicode for strings but your receiving ascii bytes so you need to read the serial data from the Adruino into a byte[] array and then use an encoder to convert the byte[] array to a unicode string.

ok, i will try it. thank a lot :)