问XML文件中值的验证

EN

private void btnNext_Click(object sender, RoutedEventArgs e, Pupil p)
{
    if (txt_Username.Text = Pupil.forename)
    {
        this.Hide();
        Display nw = new Display(theClass);
        nw.ShowDialog();
        this.Show();
    }
    MessageBox.Show("Cannot Find username");
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace PassingData
{
    public class Pupil
    {
        private string forename;
        private int score;

        public Pupil(string forename, int score)
        {
            this.forename = forename;
            this.score = score;
        }

        public Pupil()
        {
            this.forename = "Unknown";
        }

        public string Forename
        {
            get { return forename; }
            set { forename = value; }
        }

        public int Score
        {
            get { return score; }
            set { score = value; }
        }

        override public string ToString()
        {
            string output = forename + "\t" + score;
            return output;
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<ArrayOfPupil xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Pupil>
    <Forename>Andy</Forename>
    <Score>0</Score>
  </Pupil>
  <Pupil>
    <Forename>Bob</Forename>
    <Score>10</Score>
  </Pupil>
  <Pupil>
    <Forename>Carl</Forename>
    <Score>20</Score>
  </Pupil>
  <Pupil>
    <Forename>Dave</Forename>
    <Score>30</Score>
  </Pupil>
  <Pupil>
    <Forename>Eric</Forename>
    <Score>40</Score>
  </Pupil>
  <Pupil>
    <Forename>Frank</Forename>
    <Score>50</Score>
  </Pupil>
</ArrayOfPupil>