Passing a nil FolderItem to FolderItem.MoveFileTo results in a
NilObjectException. This is either a bug, or is working by design,
but is undocumented. I filed a bug report <http://
www.realsoftware.com/feedback/viewreport.php?reportid=ddaggcfz> to
which the following evaluation was added.
"This is behaving by design. You're passing in nil; what do you
expect to happen? The exception is an appropriate way to alert you of
the error."
What I would expect to happen is that the method would simply return
and set an error value in FolderItem.LastErrorCode, in keeping with
the design of the FolderItem class, as well as much of the REALbasic
framework. There is no reason to raise an exception, because it's
easy for MoveFileTo to know how to handle a nil parameter -- set an
error code and return -- and it is easy to check whether or not the
method succeeded by testing the Exists property of both the
FolderItem objects involved.
But, assuming arguendo that raising an exception is a reasonable
response, it should be documented because the raising of an exception
is then part of the method interface, as with, say,
Dictionary.Remove. And a NilObjectException is semantically ill-
chosen. According to the REALbasic documentation, "A
NilObjectException occurs when you use a property or variable to
access an object that doesn't exist.". Here, the problem is that one
has passed a parameter that the method refuses to accept; in DBC
terms, it's a precondition violation. So the proper response would
be for REALbasic to define a RuntimeException subclass that captures
such a notion, and raise an instance of it here.
Charles Yeomans
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