VB.NET 'If' statement with 'Or' co

问题:

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?

回答1:

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...

回答2:

OrElse is the short-circuited equivalent of Or