IllegalCast

General
1

My app opens a “Maintenance window” from a parent window. I now need the parent window to open any number of maintenance windows, each of which handles a different kind of data. To do this, I have an array of WeakRefs to store the various maintenance windows in. That way when I close one of the maintenance windows, its array entry can go nil and I remove it from the array. My goal is that when the user needs a new maintenance window, I don’t want them to be able to open more than one for each kind of data.

However, in the code that searches for a duplicate entry, I’m getting an IllegalCast exception and for the life of me I can’t see why.

‘WhichWnd is the parent window
‘.MyMaintWnds() is the array of WeakRefs which hold references to Maintenance windows
‘.MyCid is a category ID number that identifies the categoryID number of the data in the Maintenance window

‘The goal: If a maintenance window with this category already exists, then show it, here is the relevant code:

For i As Integer = 0 To UBound(WhichWnd.MyMaintWnds)
  
  If WhichWnd.MyMaintWnds(i)<>Nil Then 'The WeakRef in MyMaintWnds() isn't Nil
   
      'This line is OK:
    If MaintenanceWnd(WhichWnd.MyMaintWnds(i))<>Nil Then
      
      'An IllegalCast exception happens in this next line, even though .MyMaintWnds(i) is a WeakRef to an existing MaintenanceWnd:
      
      If MaintenanceWnd(WhichWnd.MyMaintWnds(i)).MyCid=WhichCat Then. <<== IllegalCast at MaintenanceWnd(

       'We found a maintenance window with the desired category, so show it.
        MaintenanceWnd(WhichWnd.MyMaintWnds(i)).Show
        Return

      End If
      
    End If
    
  End If
  
Next

Thoughts? Suggestions?

Thanks!

2

You need to use the Value of the WeakRef, so you’ll need to check that for nil too, and then cast the value

3

Ah yes, I’d forgotten that, and I didn’t see it in the documentation. Thank you for the quick catch!

4

I used a lot of WeakRefs in the web framework.

5

Wouldn’t a NilObjectException be more helpful in this situation? (I’m sure this has been discussed before and there’s probably a good reason.)