CODEWITHSUNDEEP
vb.net
Jake R
Jake R

Reputation: 116

Is Operator inconsistent behaviour

I have inconsistent behaviour of the VB Is Operator, as reproduced in a clean VB2012 Windows Executable project with code below. This makes it very difficult to debug some of my code, is this behaviour by design or is it a bug in VS/JIT?

Problem Sumary

At the breakpoint, Debug output show True, but Immediate Window command ?A Is B returns False

If I change ByRef to ByVal, the problem goes away

Public Sub Test()
    TestInner(Nothing)
End Sub

Public Sub TestInner(ByRef A As Object)
    Dim B As Object

    B = New Object
    A = B

    Debug.WriteLine(A Is B)
    'Insert breakpoint here
End Sub

I have what looks like a Compiler / Interpreter problem, has anyone seen this before?

Note that this is a simplification of my (production code) problem. If I run the exact code above, it returns True in both cases and works ok, but in my real world scenario, using fairly simple classs it fails as above.

I checked whether the classes I use have some sort of declaration problem or somehow override the Is Operator, but don't see anything weird about my classes.

I don't see how this could be anything other than a core .NET problem and am very confused!!

Upvotes: 0

Views: 86

Answers (1)

Damien_The_Unbeliever
Damien_The_Unbeliever

Reputation: 239704

I would say that this is a bug, although I can't find any specific documentation about it.

The VB (and C#) expression evaluators have been extensively re-written for the VS2015 release and I would say that it's a bug in the older expression evaluator.

If we take your code and put it in a new console application in VS2015, ?A Is B in the immediate window prints True. If, however, we change this setting:

Expression Evaluators (Image from here)

And then repeat the experiment, we do indeed get the behaviour you've observed and it prints False.

Upvotes: 1

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