public long MetaExtraData

 get return (long)(ViewState["MetaData"] ?? 0); 
 set ViewState["MetaData"] = value; 

When ViewBag doesn't contains the key 'MetaData' I get back a "can't cast to long" exception. Isn't the whole idea with ?? to handle this?

And of course, the below snippet will also throw a cast error. When checking the values of the variables: o=null

public long MetaExtraDataInt

 get
 
 object o = ViewState["MetaData"];
 return (long)(o ?? 0);
 
 set ViewState["MetaData"] = value; 

I know the problem is easy to solve, but I really do want to know why the ?? don't work on a statebag

If (int)(ViewState["MetaDataInt"] ?? 0) is throwing an InvalidCastException, it must be because your value in the statebag isn't an int.

This is because:

a boxed T can only be unboxed to T (or Nullable.) Once it is
unboxed, it’s just a value that can be cast as usual.

The above quote is taken from Representation and Identity by Eric Lippert, which I recommend you read.

So if you are storing a type that is anything other than int or int? in your "MetaDataInt" statebag entry, you will get an invalid cast exception, even if that value is something that could cast to int.

For example, if it contains a decimal, then you can only unbox it as a decimal. After that, you can cast as you normally would.

The following will work in the example scenario of unboxing a decimal:

return (int)(decimal)(ViewState["MetaDataInt"] ?? 0)

I suggest you look at the type of ViewState["MetaDataInt"] in the debugger, and alter your cast accordingly.

Edit:

I can see what's going on. In your screenshot, you're attempting to unbox to a long, but your coalesced zero is an int. This is why you're getting an exception.

The following will fix your specific problem.

return (long)(ViewState["ParentID"] ?? 0L)

Note the 0L, this is how you write a long as a numeric literal.

You're having the issue because you're coalescing the 0 int with a null object, so the result of that coalesce is also an object, in this case a boxed int. A boxed int can only be unboxed to an int, but you're attempting to unbox it to a long. We fix this by boxing a 0 long instead, so you can unbox it to a long.

Use Nullable<int> property instead of normal int to cast with null-coalescing operator:

public int? MetaExtraDataInt

 get return (int?)ViewState["MetaDataInt"] ?? 0; 
 set ViewState["MetaDataInt"] = value; 

Based from operator precedence, the null-coalescing is performed after cast to Nullable<int>, so that it returns zero when ViewState["MetaDataInt"] contains null value.

Does it work when ViewBag contains the key? Try to cast the viewstate return like:

get return (int)(ViewState["MetaDataInt"]) ?? 0);