You're confusing an object in a collection with collection itself. You still have read-only access to the collection.
The same is true for an objects in an array. You can change the object properties but not the array item it's stored in.

>Not for objects.
>You could change their properties in FOR EACH loop.
>

>FOR EACH oColumn IN thisform.Grid1.Columns
>    oColumn.ReadOnly = (oColumn.ColumnOrder % 2 == 0)
>NEXT
>