FormsAuthenticationWithUserData.cs

using Newtonsoft.Json;
using System.Web;
using System.Web.Security;

public class LoggedInUser
{
    public string FirstName { get; set; } = null;
    public bool IsAdmin { get; set; } = false;
}

public static class Authentication
{
    static void SignIn(
        HttpContextBase context,
        string emailAddress,
        bool rememberMe,
        LoggedInUser user = null)
    {
        var cookie = FormsAuthentication.GetAuthCookie(
            emailAddress.ToLower(),
            rememberMe);
        var oldTicket = FormsAuthentication.Decrypt(cookie.Value);
        var newTicket = new FormsAuthenticationTicket(
            oldTicket.Version,
            oldTicket.Name,
            oldTicket.IssueDate,
            oldTicket.Expiration,
            oldTicket.IsPersistent,
            JsonConvert.SerializeObject(user ?? new LoggedInUser()));
        
        cookie.Value = FormsAuthentication.Encrypt(newTicket);

        context.Response.Cookies.Add(cookie);
    }

    static void SignOut(HttpContextBase context)
    {
        FormsAuthentication.SignOut();
    }

    static LoggedInUser GetLoggedInUser()
    {
        if (HttpContext.Current.User?.Identity?.Name != null && HttpContext.Current.User?.Identity is FormsIdentity identity)
            return JsonConvert.DeserializeObject<LoggedInUser>(identity.Ticket.UserData);

        return new LoggedInUser();
    }
}