为什么SecurityAction.Demand总之抛出异常？

[lckboy]

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Security.Permissions;
using System.Security;
using System.Security.Principal;
using System.Web;
using System.Web.Security;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

namespace WebApplication2
{
/// <summary>
/// WebForm1 的摘要说明。
/// </summary>
public class WebForm1 : System.Web.UI.Page
{
protected System.Web.UI.WebControls.TextBox TextBox1;
protected System.Web.UI.WebControls.Button Button1;
protected System.Web.UI.WebControls.Label Label1;
protected System.Web.UI.WebControls.CheckBox CheckBox1;
protected System.Web.UI.WebControls.Label Label2;
protected System.Web.UI.WebControls.Label Label3;
protected System.Web.UI.WebControls.TextBox TextBox2;

private void Page_Load(object sender, System.EventArgs e)
{
// 在此处放置用户代码以初始化页面
}

#region Web 窗体设计器生成的代码
override protected void OnInit(EventArgs e)
{
//
// CODEGEN: 该调用是 ASP.NET Web 窗体设计器所必需的。
//
InitializeComponent();
base.OnInit(e);
}

/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.Button1.Click += new System.EventHandler(this.Button1_Click);
this.Load += new System.EventHandler(this.Page_Load);

}
#endregion

private void Button1_Click(object sender, System.EventArgs e)
{
FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(
1,
TextBox1.Text,
System.DateTime.Now,
System.DateTime.Now.AddMinutes(2),
CheckBox1.Checked,
TextBox2.Text);

FormsIdentity id = new FormsIdentity(authTicket);

string[] roles = {authTicket.UserData};

IPrincipal principal = new GenericPrincipal(id, roles);

Context.User = principal;

Label2.Text = User.Identity.Name;
Label3.Text = User.IsInRole("Student").ToString();

try
{
Text();
}
catch(SecurityException se)
{
Label1.Text = "没有权限";
}
}

[PrincipalPermission(SecurityAction.Demand, Role = "Student")]
public void Text()
{
Label1.Text = "成功分配权限";
}
}
}

[lckboy]

我在TextBox2.Text输入Student，Label3.Text就是true，这就说明了角色是Student。但是[PrincipalPermission(SecurityAction.Demand, Role = "Student")]总是抛出异常，无论有没有Role也是的

请大家指点一下，谢谢

[saucer]

IPrincipal principal = new GenericPrincipal(id, roles);
Context.User = principal;
System.Threading.Thread.CurrentPrincipal = principal;
.....

[lckboy]

感谢saucer(思归)！