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 + "','"+ EncMD5("'" + this.txtPassword.Text + "'") +"', " +
" '" + this.txtAddress.Text + "','" + this.txtTel.Text + "','" + this.txtMobile.Text + "','" + this.txtEmail.Text +"' )";