private bool CheckUsername()
        {
            var conn = SQLHelpers.GetConnection();

            conn.Open();

            var command = new SqlCommand("SELECT * FROM Bruger WHERE Brugernavn=@brugernavn",
                                         conn);

            command.Parameters.AddWithValue("@brugernavn", UserNameTextBox.Text);
            using (var reader = command.ExecuteReader())
            {
                if (reader.Read())
                {
                    conn.Close();
                    return(false);
                }
            }
            return(true);
        }