ASP.NET：为什么FormsAuthenticationTicket在认证超时后为空？

public class AuthenticationModule : IHttpModule
{
    #region IHttpModule Members
    void IHttpModule.Dispose() { }
    void IHttpModule.Init(HttpApplication application)
    {
        application.AuthenticateRequest += new EventHandler(this.context_AuthenticateRequest);
    }
    #endregion


    /// <summary>
    /// Inspect the auth request...
    /// </summary>
    /// <remarks>See "How To Implement IPrincipal" in MSDN</remarks>
    private void context_AuthenticateRequest(object sender, EventArgs e)
    {
        HttpApplication a = (HttpApplication)sender;
        HttpContext context = a.Context;

        // Extract the forms authentication cookie
        string cookieName = FormsAuthentication.FormsCookieName;
        HttpCookie authCookie = context.Request.Cookies[cookieName]; // no longer a forms cookie in this array once timeout has expired

        if (authCookie != null)
        {
            FormsAuthenticationTicket authTicket = FormsAuthentication.Decrypt(authCookie.Value);
            DateTime expirationTime = authTicket.Expiration;
            // check if previously authenticated session is now dead
            if (authTicket != null && authTicket.Expired)
            {
                // send them a Response indicating that they've expired.
            }
        }
    }
}