ASP.Net C# รบกวนสอบถามเกี่ยวกับการเรียกใช้ class และการเขียนเรียกใช้ Function หน่อยครับ

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Text;
using System.Security.Cryptography;//**md5**//

namespace User_Login
{
    public partial class Register : System.Web.UI.Page
    {
        //////** Password **///////
        public string EncMD5(string input)
        {
            MD5 md5Hashfc = MD5.Create();
            byte[] data = md5Hashfc.ComputeHash(Encoding.Default.GetBytes(input));
            StringBuilder sBuilder = new StringBuilder();
            int i = 0;
            for (i = 0; i <= data.Length - 1; i++)
            {
                sBuilder.Append(data[i].ToString("x2"));
            }
            return sBuilder.ToString();
        }
        /////**End Password **//////

protected void btnRegister_Click(object sender, EventArgs e)
        {

            SqlConnection objConn = new SqlConnection();
            SqlCommand objCmd = new SqlCommand();
            String strConnString, strSQL;

            //int intCount = 0;

            //*** Open Connection ***'
            strConnString = "Data Source=OFF;Initial Catalog=customer;Integrated Security=True";
            objConn = new SqlConnection();
            objConn.ConnectionString = strConnString;
            objConn.Open();


            strSQL = "INSERT INTO register (username,lastname,password,address,tel,mobile,email) " +
            " VALUES " +
            " ('" + this.txtUsername.Text + "','" + this.txtLastname.Text + "','"+ [font=Verdana]EncMD5[/font]("'" + this.txtPassword.Text + "'") +"', " +
            " '" + this.txtAddress.Text + "','" + this.txtTel.Text + "','" + this.txtMobile.Text + "','" + this.txtEmail.Text +"' )";