Casting a Null

Found an interesting tidbit today. There is an ASP.NET page that is using session variables. The user has visited the page, and populated the session. But then the user lingers, and the session times out on a page that is not protected. The user then clicks a button, and the following line of C# code executes:

User foo = (User)Session[“bar”];

So what should happen here? An Exception? That’s what I would have thought. But not true. Since the session has expired, the session variable is now null, and you can cast a null into another data type having a value of null. Just as long as that type is nullable. So the line above is the same as:

User foo = (User)null;

which is perfectly valid.

I got the null reference exception down the line when trying to access a property of foo. So once again, defensive programming is the rule. Check your session variables for null!