VB.NET带有'或'条件的'If'语句双方都经过评估了吗? [英] VB.NET 'If' statement with 'Or' conditional has both sides evaluated?

问题描述

快速问题，最快和最简单的答案可能就是重新排列相关代码，但是让我们看看...

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

所以我有一个 If 语句(一段代码，它是用C#编写的完整工作解决方案的一部分)，是使用VB.NET重写的.我知道VB.NET IIf(a，b，c)方法会同时评估 b 和 a ，而不管第一次评估的真实性如何，但在我的标准构造中，似乎也是如此:

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

或者，更恰当地说:

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

在这里，如果 example 是 Nothing ( null )，那么我仍然会收到 NullReferenceException -这是我的吗?是错误的，还是我只需要忍受VB.NET的一时冲动?

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?

推荐答案

这是您的错误"，这就是 Or 已定义，因此这是您应该期望的行为:

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

在布尔比较中，或"运算符始终对两个表达式求值，这可能包括进行过程调用.OrElse运算符(Visual Basic)执行短路，这意味着如果expression1为True，则不对expression2求值.

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.

但是您不必忍受它.您可以使用 OrElse 来获得-循环行为.

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

所以您可能想要:

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

我不能说它读得很好，但是它应该可以工作...

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

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