List<string> roles = new List<string>();

if (isAdmin) {
    roles.Add("Admin");
} 
roles.Add("Guest");

FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(
    1, 
    model.UserId, 
    DateTime.Now, 
    DateTime.Now.AddDays(30),    // 设置记住登录的时间（这里是30天）
    true,
    String.Join(",", roles));    // 设置用户角色
HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, FormsAuthentication.Encrypt(ticket));
// 设置cookie的过期时间（这里是50年）
cookie.Expires = DateTime.Now.AddYears(50);
Response.Cookies.Add(cookie);

protected void Application_AuthenticateRequest(object sender, EventArgs e) {
    // 取得认证Check的Cookie
    HttpCookie cookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName];

    if (cookie == null) return;

    // 解密
    FormsAuthenticationTicket ticket = null;
    try {
        ticket = FormsAuthentication.Decrypt(cookie.Value);
    }
    catch (Exception) {
        return;
    }
    if (ticket == null) return;

    // 取得ticket.UserData中设定的角色
    string[] roles = ticket.UserData.Split(new char[] { ‘,‘ });

    // From认证中，使用IPrincipal对象中的GenericPrincipal类。
    // 该类由表示资格情报的FormsIdentity类和角色信息(string[]对象)组成。
    FormsIdentity identity = new FormsIdentity(ticket);
    GenericPrincipal principal = new GenericPrincipal(identity, roles);

    // 把FormsIdentity赋值到Context.User中
    // 可以从Page.User中取得该值
    Context.User = principal;
}

// 允许匿名访问
[AllowAnonymous]
public class HomeController : Controller
{
    ......
}

// 只允许登录用户访问
[Authorize]
public class SampleController : Controller 
{
    ......
}

// 只允许具有"Admin"角色的用户访问
[Authorize(Roles = "Admin")]
public class SampleController : Controller 
{
    ......
}

// 该特性同样可用于Action