在VB.NET IF语句的评价
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20-08-2019 - |
题
有关以下if语句在VB.NET,这将是在其中的条件将被评估的序列?
<强>例1:强>
If ( condition1 AND condition2 AND condition3 )
.
.
End If
<强>情况2:强>
If ( condition1 OR condition2 OR condition3 )
.
.
End If
<强>情况3:强>
If ( condition1 OR condition2 AND condition3 OR condition4)
.
.
End If
解决方案
VB.NET是从C程序员的观点来看很奇怪的兽。作为Gerrie在不同的答复中提到,所有这三个条件在充分评估整体非短路。 AndAlso和OrElse运算可以节约你的时间,如果这就是你想要的。
至于最后的如果,评价的顺序如下:
If ((condition1 OR (condition2 AND condition3)) OR condition4)
作为一个经验法则:如果有任何的ambiguitiy,用括号指定计算顺序明确
其他提示
VB.Net评估所有条件,所以为了在这里并不重要。
如果你想使用短路,使用AndAlso和OrElse运算的关键字。
这是不完全应答手头上的问题,因为它已经回答了,我觉得有必要扩大对“OrElse运算”关键字,由Anton提到的,以及它是相对的:“AndAlso”,因为有人登陆这个问题我其实想对这些解释。
“OrElse运算”和“AndAlso”如果你需要明令评价是非常有用的。
不像“或”,“与”,都表达进行评估,如果语句可以跳过剩下的表达式,如果第一个表达式为需要的值,当你利用“OrElse运算”或“AndAlso”的。
在下面的情况下,表达式的顺序也适用于“OrElse运算”从左到右,但也并不总是评估取决于先前值的结果的所有表达式:
if( Expression1 orelse Expression2 orelse Expression3 )
' Code...
End If
如果表达式1为真,则表达式2和3将被忽略。 如果表达式1如果为假,则表达式2被评估,然后表达式3如果表达式2计算结果为True。
If (False OrElse True OrElse False ) then
' Expression 1 and 2 are evaluated, Expression 3 is ignored.
End If
If (True OrElse True OrElse True ) then
' only Expression 1 is evaluated.
End If
If (True OrElse True OrElse True ) then
' only Expression 1 is evaluated.
End If
If (False OrElse False OrElse True ) then
' All three expressions are evaluated
End If
的 'OrElse运算' 的使用的替代示例:
If( (Object1 is Nothing) OrElse (Object2 is Nothing) OrElse (Object3 is Nothing) ) then
' At least one of the 3 objects is not null, but we dont know which one.
' It is possible that all three objects evaluated to null.
' If the first object is null, the remaining objects are not evaluated.
' if Object1 is not NULL and Object2 is null then Object3 is not evaluated
Else
' None of the objects evaluated to null.
Endif
在上面的例子中是相同的,如下所示:
If(Object1 Is Nothing)
' Object 1 is Nothing
Return True
Else
If(Object2 Is Nothing)
' Object 2 is Nothing
Return True
Else
If(Object3 Is Nothing)
' Object 3 is Nothing
Return True
Else
' One of the objects evaluate to null
Return False
End If
End If
End If
还有的 AndALso 强>关键字:
' If the first expression evaluates to false, the remaining two expressions are ignored
If( Expression1 AndAlso Expression2 AndAlso Expression3 ) Then ...
哪些是可用的这样的:
If( (Not MyObject is Nothing) AndAlso MyObject.Enabled ) Then ...
' The above will not evaluate the 'MyObject.Enabled' if 'MyObject is null (Nothing)'
' MyObject.Enabled will ONLY be evaluated if MyObject is not Null.
' If we are here, the object is NOT null, and it's Enabled property evaluated to true
Else
' If we are here, it is because either the object is null, or it's enabled property evaluated to false;
End If
不同于 '与',像“OR'-将始终评估所有表达式:
If( (Not MyObject is Nothing) And MyObject.Enabled ) Then ...
' MyObject.Enabled will ALWAYS be evaluated, even if MyObject is NULL,
' --- which will cause an Exception to be thrown if MyObject is Null.
End If
但是,由于为了能够具有的效果与“OrElse运算”和“AndAlso”,评估天气一个对象为空,应先进行,如在上述实例。
下面将引起一个异常,如果“为MyObject”为空
Try
If( MyObject.Enabled AndAlso (Not MyObject is Nothing) ) Then ...
' This means that first expression MyObject.Enabled Was evaluated to True,
' Second Expression also Evaluated to True
Else
' This means MyObject.Enabled evaluated to False, thus also meaning the object is not null.
' Second Expression "Not MyObject is Nothing" was not evaluated.
End If
Catch(e as Exception)
' An exception was caused because we attempted to evaluate MyObject.Enabled while MyObject is Nothing, before evaluating Null check against the object.
End Try
尽请评论,如果我犯了一个错误或错字在这里,因为我早上写了这个在上午05点16分后2天没有睡觉了。