Invalid cast when getting custom IPrincipal from HttpContext

Question

After researching FormsAuthentication for a few days, I decided to store a serialized object in the FormsAuth cookie's UserData property and use a custom IPrincipal object for the HttpContext.Current.User.

Most of the guides I've found say to cast the IPrincipal object to your object. I get an invalid cast exception every time though. What am I doing wrong?

MyUserData

public class MyUserData
{
    public long UserId { get; set; }
    public string Username { get; set; }
    public bool IsSuperUser { get; set; }
    public string UnitCode { get; set; }
    public string EmailAddress { get; set; }
    public List<string> Roles { get; set; }

    // Serialize    
    public override string ToString()
    {
        JavaScriptSerializer serializer = new JavaScriptSerializer();
        string result = serializer.Serialize(this);
        return result;
    }

    // Deserialize
    public static MyUserData FromString(string text)
    {
        JavaScriptSerializer serializer = new JavaScriptSerializer();
        return serializer.Deserialize<MyUserData>(text);
    }
}

CustomPlatformPrincipal

public class MyCustomPrincipal : IPrincipal
{
    public MyUserData MyUserData { get; set; }
    public IIdentity Identity { get; private set; }

    public MyCustomPrincipal(MyUserData myUserData)
    {
        MyUserData = myUserData;
        Identity = new GenericIdentity(myUserData.Username);
    }

    public bool IsInRole(string role)
    {
        return MyUserData.Roles.Contains(role);
    }
}

Global.asax.cs

protected void Application_AuthenticateRequest(Object sender, EventArgs e)
    {
        HttpCookie authCookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName];
        if (authCookie == null || authCookie.Value == "")
        {
            return;
        }

        FormsAuthenticationTicket authTicket;
        try
        {
            authTicket = FormsAuthentication.Decrypt(authCookie.Value);
        }
        catch
        {
            return;
        }

        if (Context.User != null)
        {
            // the from string deserializes the data
            MyUserData myUserData = MyUserData.FromString(authTicket.UserData);
            Context.User = new MyCustomPrincipal(myUserData);
        }
    }

My Page

var myUserData = ((MyCustomPrincipal)(HttpContext.Current.User)).MyUserData;
// invalid cast exception (can't cast IPrincipal to MyCustomPrincipal)

Article I was following: http://primaryobjects.com/CMS/Article147.aspx

So it seems the only way I could get my data is to decrypt the auth cookie, then deserialize the authCookie's userData string.

Any suggestions?

Update

Tried following the suggestions on this SO question: Implementing a Custom Identity and IPrincipal in MVC

Code is below, but it didn't work.

[Serializable]
public class MyCustomPrincipal : IPrincipal, ISerializable
{
    public CustomUserData CustomUserData { get; set; }
    public IIdentity Identity { get; private set; }

    //public MyCustomPrincipal (IIdentity identity) { Identity = identity; }

    public MyCustomPrincipal(CustomUserData customUserData)
    {
        CustomUserData = customUserData;
        Identity = new GenericIdentity(customUserData.Username);
    }

    public bool IsInRole(string role)
    {
        return PlatformUserData.Roles.Contains(role);
    }


    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        if (context.State == StreamingContextStates.CrossAppDomain)
        {
            MyCustomPrincipal principal = new MyCustomPrincipal (this.CustomUserData );
            info.SetType(principal.GetType());

            System.Reflection.MemberInfo[] serializableMembers;
            object[] serializableValues;

            serializableMembers = FormatterServices.GetSerializableMembers(principal.GetType());
            serializableValues = FormatterServices.GetObjectData(principal, serializableMembers);

            for (int i = 0; i < serializableMembers.Length; i++)
            {
                info.AddValue(serializableMembers[i].Name, serializableValues[i]);
            }
        }
        else
        {
            throw new InvalidOperationException("Serialization not supported");
        }
    }
}

Answer 1

Did you run in the debug mode? You can put break point on HttpContext.Current.User, you will see what type the user was at that moment. And from your Application_AuthenticateRequest method, there is no guarantee that the User will be your expected type. There are many exit points before reaching your custom type setup. Even this code: Context.User != null. It was wrong with your expectation. I have not gone through the detail of the Context.User, however, in term of your context, you were expecting the Context.User was your custom user. So the valid check should be:

var custom = Context.Current as MyCustomPrinciple;
if(custom == null)
{
// Your construct code here.
}

My strongly suggestion is: you need to go in debug mode, to see exactly what was going on.