gistfile1.cs

using System;
using System.Security.Principal;
using System.Web.Mvc;
using System.Web.Security;
using MvcApplication7.Models;

namespace MvcApplication7.Controllers
{
    public class CurrentUserFilter : IAuthorizationFilter
    {
        private readonly IUserRepository _repository;

        public CurrentUserFilter(IUserRepository repository)
        {
            _repository = repository;
        }

        public void OnAuthorization(AuthorizationContext filterContext)
        {
            var authCookie = filterContext.HttpContext.Request.Cookies[FormsAuthentication.FormsCookieName];
            if (authCookie == null)
                return;

            var ticket = FormsAuthentication.Decrypt(authCookie.Value); // Contains userId from FormsAuthentication.SignUp(userId, true);
            var userId = Convert.ToInt32(ticket.Name);
            var user = _repository.GetUserById(userId);

            var principal = new MyUserPrincipal(new MyUserIdentity(user.Id, user.Username, user.AvatarUrl));
            filterContext.HttpContext.User = principal;
        }
    }


    public class MyUserPrincipal : IPrincipal
    {
        private readonly MyUserIdentity _identity;
        public MyUserPrincipal(MyUserIdentity identity)
        {
            _identity = identity;
        }

        public bool IsInRole(string role)
        {
            return true; // or you know, put your logic here mate!
        }

        public IIdentity Identity
        {
            get { return _identity; }
        }
    }

    public class MyUserIdentity : IIdentity
    {
        public int UserId { get; set; }
        public string Username { get; set; }
        public string AvatarUrl { get; set; }

        public MyUserIdentity(int userId, string username, string avatarUrl)
        {
            UserId = userId;
            Username = username;
            AvatarUrl = avatarUrl;
        }

        public string Name
        {
            get { return Username; }
        }

        public string AuthenticationType
        {
            get { return "Custom"; }
        }

        public bool IsAuthenticated
        {
            get { return string.IsNullOrWhiteSpace(Username) == false; }
        }
    }
}