string roles = "Job_System";
FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(1, strAccount, DateTime.Now, DateTime.Now.AddMinutes(30), false, roles);
string encryptedTicket = FormsAuthentication.Encrypt(authTicket); //加密 
HttpCookie UserCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
Context.Response.Cookies.Add(UserCookie); //写入

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完整教程
ASP.NET Cookie 概述
http://msdn.microsoft.com/zh-cn/library/ms178194%28VS.80%29.aspx