IPrinciple Implementation

CustomPrinciple.cs

//Create the interface
interface ICustomPrincipal : IPrincipal
{
    int Id { get; set; }
    string FirstName { get; set; }
    string LastName { get; set; }
}
//CustomPrincipal
public class CustomPrincipal : ICustomPrincipal
{
    public IIdentity Identity { get; private set; }
    public bool IsInRole(string role) { return false; }

    public CustomPrincipal(string email)
    {
        this.Identity = new GenericIdentity(email);
    }

    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
//CustomPrincipalSerializeModel - for serializing custom information into userdata field in FormsAuthenticationTicket object.
public class CustomPrincipalSerializeModel
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
//LogIn method - setting up a cookie with custom information
if (Membership.ValidateUser(viewModel.Email, viewModel.Password))
{
    var user = userRepository.Users.Where(u => u.Email == viewModel.Email).First();

    CustomPrincipalSerializeModel serializeModel = new CustomPrincipalSerializeModel();
    serializeModel.Id = user.Id;
    serializeModel.FirstName = user.FirstName;
    serializeModel.LastName = user.LastName;

    JavaScriptSerializer serializer = new JavaScriptSerializer();

    string userData = serializer.Serialize(serializeModel);

    FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(
             1,
             viewModel.Email,
             DateTime.Now,
             DateTime.Now.AddMinutes(15),
             false,
             userData);

    string encTicket = FormsAuthentication.Encrypt(authTicket);
    HttpCookie faCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encTicket);
    Response.Cookies.Add(faCookie);

    return RedirectToAction("Index", "Home");Cus
}
//Global.asax.cs - Reading cookie and replacing HttpContext.User object, this is done by overriding PostAuthenticateRequest
protected void Application_PostAuthenticateRequest(Object sender, EventArgs e)
{
    HttpCookie authCookie = Request.Cookies[FormsAuthentication.FormsCookieName];

    if (authCookie != null)
    {
        FormsAuthenticationTicket authTicket = FormsAuthentication.Decrypt(authCookie.Value);

        JavaScriptSerializer serializer = new JavaScriptSerializer();

        CustomPrincipalSerializeModel serializeModel = serializer.Deserialize<CustomPrincipalSerializeModel>(authTicket.UserData);

        CustomPrincipal newUser = new CustomPrincipal(authTicket.Name);
        newUser.Id = serializeModel.Id;
        newUser.FirstName = serializeModel.FirstName;
        newUser.LastName = serializeModel.LastName;

        HttpContext.Current.User = newUser;
    }
}
/*Access in Razor views
@((User as CustomPrincipal).Id)
@((User as CustomPrincipal).FirstName)
@((User as CustomPrincipal).LastName)
and in code:

    (User as CustomPrincipal).Id
    (User as CustomPrincipal).FirstName
    (User as CustomPrincipal).LastName
I think the code is self-explanatory. If it isn't, let me know.*/

//Additionally to make the access even easier you can create a base controller and override the returned User object (HttpContext.User):

public class BaseController : Controller
{
    protected virtual new CustomPrincipal User
    {
        get { return HttpContext.User as CustomPrincipal; }
    }
}
//and then, for each controller:

public class AccountController : BaseController
{
    // ...
}
/*which will allow you to access custom fields in code like this:

User.Id
User.FirstName
User.LastName*/

//But this will not work inside views. For that you would need to create a custom WebViewPage implementation:

public abstract class BaseViewPage : WebViewPage
{
    public virtual new CustomPrincipal User
    {
        get { return base.User as CustomPrincipal; }
    }
}

public abstract class BaseViewPage<TModel> : WebViewPage<TModel>
{
    public virtual new CustomPrincipal User
    {
        get { return base.User as CustomPrincipal; }
    }
}
/*Make it a default page type in Views/web.config:

<pages pageBaseType="Your.Namespace.BaseViewPage">
  <namespaces>
    <add namespace="System.Web.Mvc" />
    <add namespace="System.Web.Mvc.Ajax" />
    <add namespace="System.Web.Mvc.Html" />
    <add namespace="System.Web.Routing" />
  </namespaces>
</pages>
and in views, you can access it like this:

@User.FirstName
@User.LastName
*/