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Grant Thomas
Grant Thomas

Reputation: 45058

VB.NET 'If' statement with 'Or' conditional has both sides evaluated?

Quick question, of which the quickest and easiest answer may well be to rearrange related code, but let's see...

So I have an If statement (a piece of code which is a part of a full working solution written in C#) rewritten using VB.NET. I am aware the VB.NET IIf(a, b, c) method evaluates both b and a regardless of the trueness of the first evaluation, but this seems to be the case in my standard construct, too:

If (example Is Nothing Or example.Item IsNot compare.Item) Then
    'Proceed
End If

Or, rather, more appropriately:

If (example Is Nothing Or Not example.Item = compare.Item) Then
    'Proceed
End If

Here, if example is Nothing (null) then I still get an NullReferenceException - is this my fault, or is it something I just have to endure at the whim of VB.NET?

Upvotes: 30

Views: 123492

Answers (2)

Jon Skeet
Jon Skeet

Reputation: 1502286

It's your "fault" in that that's how Or is defined, so it's the behaviour you should expect:

In a Boolean comparison, the Or operator always evaluates both expressions, which could include making procedure calls. The OrElse Operator (Visual Basic) performs short-circuiting, which means that if expression1 is True, then expression2 is not evaluated.

But you don't have to endure it. You can use OrElse to get short-circuiting behaviour.

So you probably want:

If (example Is Nothing OrElse Not example.Item = compare.Item) Then
    'Proceed
End If

I can't say it reads terribly nicely, but it should work...

Upvotes: 57

Richard Szalay
Richard Szalay

Reputation: 84784

OrElse is the short-circuited equivalent of Or

Upvotes: 23

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