VB check for Null Reference when passing ByRef

Question

I have a function that accepts a String by reference:

Function Foo(ByRef input As String)

If I call it like this:

Foo(Nothing)

I want it to do something different than if I call it like this:

Dim myString As String = Nothing
Foo(myString)

Is it possible to detect this difference in the way the method is called in VB .NET?

Edit

To clarify why the heck I would want to do this, I have two methods:

Function Foo()
  Foo(Nothing)
End Function

Function Foo(ByRef input As String)
  'wicked awesome logic here,  hopefully
End Function

All the logic is in the second overload, but I want to perform a different branch of logic if Nothing was passed into the function than if a variable containing Nothing was passed in.

Because I have an overload of a function, but the overload calls to the other function overload and passes Nothing for that parameter. — Brandon Montgomery, Jun 06 '11 at 20:24
@Brandon: Yes, but in that case, you shouldn't care which way this was created. The function should just run knowing that it's receiving `Nothing` as input. — Reed Copsey, Jun 06 '11 at 20:28
@Brandon: If you truly need different behavior when your overload is called, you should put that in the overloaded function. — Reed Copsey, Jun 06 '11 at 20:29
I've edited the question to clarify why I would want to do this. — Brandon Montgomery, Jun 06 '11 at 20:33
@Brandon - your last sentence is what I was questioning. WHY do you care whether Nothing was passed in vs. a variable containing nothing? — Reed Copsey, Jun 06 '11 at 20:42
@Brandon: That being said, my answer still stands - this isn't possible. You're passing a copy of a string reference into the method - internally, it's exactly the same either way according to the CLR. — Reed Copsey, Jun 06 '11 at 20:43
So a null reference is the same thing as a reference to a null value? — Brandon Montgomery, Jun 06 '11 at 20:52
@Brandon: Yes. In both cases, it's an instance of a reference with a value of "0", basically, saying it's pointing to Nothing. — Reed Copsey, Jun 06 '11 at 21:01

score 6 · Accepted Answer · answered Jun 06 '11 at 18:53

6

No. In either case, the method "sees" a reference to a string (input) which is pointing to nothing.

From the method's point of view, these are identical.

answered Jun 06 '11 at 18:53

Reed Copsey

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Thanks for all the clarification, Reed. – Brandon Montgomery Jun 07 '11 at 01:42

score 0 · Answer 2 · edited Jun 09 '11 at 07:31

0

You could add a Null Reference check either:

1) prior to calling the function

If myString IsNot Nothing Then 
     Foo(myString)
End If

2) or inside the function

Function Foo(ByRef input As String)
    If input Is Nothing Then
        Rem Input is null
    Else
        Rem body of function
    End If
End Function

edited Jun 09 '11 at 07:31

user774411

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answered Jun 06 '11 at 19:14

Adam Speight

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VB check for Null Reference when passing ByRef

2 Answers2