Visual Basic .NET Language => Kurzschlussoperatoren (AndAlso

Syntax

result = expression1 AndAlso expression2
result = expression1 oder Else expression2

Parameter

Parameter	Einzelheiten
Ergebnis	Erforderlich. Jeder boolesche Ausdruck. Das Ergebnis ist das boolesche Ergebnis des Vergleichs der beiden Ausdrücke.
expression1	Erforderlich. Jeder boolesche Ausdruck.
expression2	Erforderlich. Jeder boolesche Ausdruck.

Bemerkungen

'AndAlso' und 'OrElse' sind ShortCircuiting- Operatoren. Dies bedeutet, dass die Ausführung kürzer ist, da der Compiler nicht alle Ausdrücke in einem booleschen Vergleich auswertet, wenn der erste das gewünschte Ergebnis liefert.

Und auch Verwendung

' 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.

Oder Else Nutzung

' 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

NullReferenceException vermeiden

7,0

Oder Else

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

Und auch

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

In Visual Basic 14.0 wurde der bedingungslose Operator " AndAlso eingeführt , mit dem die Funktionen sauberer geschrieben werden können, um das Verhalten der AndAlso Version des Beispiels nachzuahmen.

Modified text is an extract of the original Stack Overflow Documentation

Lizenziert unter CC BY-SA 3.0

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Visual Basic .NET Language
Kurzschlussoperatoren (AndAlso - OrElse)

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