Operador OrElse (Visual Basic)

Artigo
08/08/2011

Performs short-circuiting inclusive logical disjunction on two expressions.

result = expression1 OrElse expression2

Parts

result
Required. Any Boolean expression.
expression1
Required. Any Boolean expression.
expression2
Required. Any Boolean expression.

Comentários

A logical operation is said to be short-circuiting if the compiled code can bypass the evaluation of one expression depending on the result of another expression. If the result of the first expression evaluated determines the final result of the operation, there is no need to evaluate the second expression, because it cannot change the final result. Short-circuiting can improve performance if the bypassed expression is complex, or if it involves procedure calls.

If either or both expressions evaluate to True, result is True. The following table illustrates how result is determined.

If expression1 is	And expression2 is	The value of result is
True	(not evaluated)	True
False	True	True
False	False	False

Data Types

The OrElse operator is defined only for the Tipo de dados booleanos (Visual Basic). Visual Basic converts each operand as necessary to Boolean and performs the operation entirely in Boolean. If you assign the result to a numeric type, Visual Basic converts it from Boolean to that type. This could produce unexpected behavior. Por exemplo, 5 OrElse 12 resulta em –1 quando convertido em Integer.

Overloading

The Operador Or (Visual Basic) and the Operador IsTrue (Visual Basic) can be overloaded, which means that a class or structure can redefine their behavior when an operand has the type of that class or structure. Overloading the Or and IsTrue operators affects the behavior of the OrElse operator. If your code uses OrElse on a class or structure that overloads Or and IsTrue, be sure you understand their redefined behavior. For more information, see Procedimentos de operador (Visual Basic).

Exemplo

The following example uses the OrElse operator to perform logical disjunction on two expressions. The result is a Boolean value that represents whether either of the two expressions is true. If the first expression is True, the second is not evaluated.

Dim a As Integer = 10
Dim b As Integer = 8
Dim c As Integer = 6
Dim firstCheck, secondCheck, thirdCheck As Boolean
firstCheck = a > b OrElse b > c
secondCheck = b > a OrElse b > c
thirdCheck = b > a OrElse c > b

The preceding example produces results of True, True, and False respectively. In the calculation of firstCheck, the second expression is not evaluated because the first is already True. However, the second expression is evaluated in the calculation of secondCheck.

The following example shows an If...Then statement containing two procedure calls. If the first call returns True, the second procedure is not called. This could produce unexpected results if the second procedure performs important tasks that should always be performed when this section of the code runs.

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