Значение строк пропадает - C#

  public class Emloyee
{
        public string[] FName = new string[256];
        public string[] SName = new string[256];
        public string[] ManagerFN = new string[256];
        public string[] ManagerSN = new string[256];
}

emp = new Emloyee();
                emp.FName[num] = FNameTxtBox.Text;
                emp.SName[num] = SNameTxtBox.Text;
                emp.ManagerFN[num] = ManagerFNTxtBox.Text;
                emp.ManagerSN[num] = ManagerSNTxtBox.Text;
                listBox.Items.Add(FNameTxtBox.Text + " " + SNameTxtBox.Text);
                num += 1;