asp.net中使用MD5加密出错 - .NET技术 / ASP.NET

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.ComponentModel;
using System.Xml.Linq;
using System.Data.SqlClient;

public partial class user_a_add : System.Web.UI.Page
{
  protected void Page_Load(object sender, EventArgs e)
  {
  if (!IsPostBack)
  {
  if (Session["user_a"] != null || Session["user_c"] != null || Session["user_e"] != null)
  {

  }
  else
  { Response.Write("<script language=javascript>alert('您还没有登陆');window.window.location.href='../login.aspx';</script>"); }
  }
  }
  protected void btn_Click(object sender, EventArgs e)
  {
  string id = TextBox1.Text.ToString().Trim();
  string name = TextBox2.Text.ToString().Trim();
  string sex = TextBox3.Text.ToString().Trim();
  string age = TextBox4.Text.ToString().Trim();
  string did = TextBox5.Text.ToString().Trim();