Testing for NullReferenceException (Not Testing for Nothing)

Question

I have a class someone else wrote for which I don't have the source code. It has a property UM that is backed by _UM, a string. In some circumstances _UM is Nothing. I would expect that UM would be Nothing too, but when I inspect (using Quick Watch) the property it shows as a NullReferenceException. When I try testing for Nothing I get a NullReferenceException thrown in my main code. How can I catch this condition so I can handle it properly?

If Foo.UM Is Nothing Then
    DoSomething()
End If

...throws a NullReferenceException.

The exception may be thrown because Foo is Nothing.

Oded
– Oded

2012-04-17 20:48:10 +00:00
Commented Apr 17, 2012 at 20:48 — Oded
– Oded, Commented Apr 17, 2012 at 20:48
Foo isn't nothing. Other properties I can see.

cjbarth
– cjbarth

2012-04-17 20:53:28 +00:00
Commented Apr 17, 2012 at 20:53 — cjbarth
– cjbarth, Commented Apr 17, 2012 at 20:53
try ... catch NullReferenceException ...

Igby Largeman
– Igby Largeman

2012-04-17 21:01:34 +00:00
Commented Apr 17, 2012 at 21:01 — Igby Largeman
– Igby Largeman, Commented Apr 17, 2012 at 21:01

Magnus · Accepted Answer · 2012-04-17 20:56:48Z

1

The property might do more than just return the _UM field. Probably uses it somehow and does not take into account that it might be null. You could do something like this to handle it:

Dim obj = Nothing
Try
    obj = Foo.UM
Catch ex As NullReferenceException
End Try
If obj Is Nothing Then
    DoSomething()
End If

edited Apr 17, 2012 at 20:56

answered Apr 17, 2012 at 20:50

Magnus

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3 Comments

cjbarth Over a year ago

I would agree that the property probably does something besides just return _UM, however, I don't particularly care. I just need to test and take action. Any thoughts on how to do that?

cjbarth Over a year ago

I was hoping to avoid that as it is quite a bit of overhead to process an exception...oh well. This class is quite lousy in other areas, I shouldn't be surprised. Thanks for your suggestion.

Magnus Over a year ago

You can always decompile the external dll with Reflector for example and see what is actually going on in the property.

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