Visual Basic .NET Language
Операторы короткого замыкания (AndAlso - OrElse)
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Синтаксис
- result = expression1 AndAlso expression2
- result = expression1 OrElse expression2
параметры
| параметр | подробности |
|---|---|
| результат | Необходимые. Любое булево выражение. Результатом является логический результат сравнения двух выражений. |
| выражение1 | Необходимые. Любое булево выражение. |
| выражения2 | Необходимые. Любое булево выражение. |
замечания
«AndAlso» и «OrElse» - это операторы ShortCircuiting, что означает, что выполнение короче, потому что компилятор не оценивает все выражения в логическом сравнении, если первый дает ожидаемый результат.
AndAlso Использование
' Sometimes we don't need to evaluate all the conditions in an if statement's boolean check.
' Let's suppose we have a list of strings:
Dim MyCollection as List(Of String) = New List(of String)()
' We want to evaluate the first value inside our list:
If MyCollection.Count > 0 And MyCollection(0).Equals("Somevalue")
Console.WriteLine("Yes, I've found Somevalue in the collection!")
End If
' If MyCollection is empty, an exception will be thrown at runtime.
' This because it evaluates both first and second condition of the
' if statement regardless of the outcome of the first condition.
' Now let's apply the AndAlso operator
If MyCollection.Count > 0 AndAlso MyCollection(0).Equals("Somevalue")
Console.WriteLine("Yes, I've found Somevalue in the collection!")
End If
' This won't throw any exception because the compiler evaluates just the first condition.
' If the first condition returns False, the second expression isn't evaluated at all.
Использование OrElse
' The OrElse operator is the homologous of AndAlso. It lets us perform a boolean
' comparison evaluating the second condition only if the first one is False
If testFunction(5) = True OrElse otherFunction(4) = True Then
' If testFunction(5) is True, otherFunction(4) is not called.
' Insert code to be executed.
End If
Избегание исключения NullReference
7,0
OrElse
Sub Main()
Dim elements As List(Of Integer) = Nothing
Dim average As Double = AverageElementsOrElse(elements)
Console.WriteLine(average) ' Writes 0 to Console
Try
'Throws ArgumentNullException
average = AverageElementsOr(elements)
Catch ex As ArgumentNullException
Console.WriteLine(ex.Message)
End Try
End Sub
Public Function AverageElementsOrElse(ByVal elements As IEnumerable(Of Integer)) As Double
' elements.Count is not called if elements is Nothing so it cannot crash
If (elements Is Nothing OrElse elements.Count = 0) Then
Return 0
Else
Return elements.Average()
End If
End Function
Public Function AverageElementsOr(ByVal elements As IEnumerable(Of Integer)) As Double
' elements.Count is always called so it can crash if elements is Nothing
If (elements Is Nothing Or elements.Count = 0) Then
Return 0
Else
Return elements.Average()
End If
End Function
7,0
А также
Sub Main()
Dim elements As List(Of Integer) = Nothing
Dim average As Double = AverageElementsAndAlso(elements)
Console.WriteLine(average) ' Writes 0 to Console
Try
'Throws ArgumentNullException
average = AverageElementsAnd(elements)
Catch ex As ArgumentNullException
Console.WriteLine(ex.Message)
End Try
End Sub
Public Function AverageElementsAndAlso(ByVal elements As IEnumerable(Of Integer)) As Double
' elements.Count is not called if elements is Nothing so it cannot crash
If (Not elements Is Nothing AndAlso elements.Count > 0) Then
Return elements.Average()
Else
Return 0
End If
End Function
Public Function AverageElementsAnd(ByVal elements As IEnumerable(Of Integer)) As Double
' elements.Count is always called so it can crash if elements is Nothing
If (Not elements Is Nothing And elements.Count > 0) Then
Return elements.Average()
Else
Return 0
End If
End Function
14,0
Visual Basic 14.0 представила нулевой оператор условности , позволяющий более AndAlso переписать функции, имитируя поведение версии AndAlso в примере.
Modified text is an extract of the original Stack Overflow Documentation
Лицензировано согласно CC BY-SA 3.0
Не связан с Stack Overflow
