AndAlso Operator (Visual Basic)

Artikkeli
10/12/2021

Performs short-circuiting logical conjunction on two expressions.

Syntax

result = expression1 AndAlso expression2

Parts

Term	Definition
`result`	Required. Any `Boolean` expression. The result is the `Boolean` result of comparison of the two expressions.
`expression1`	Required. Any `Boolean` expression.
`expression2`	Required. Any `Boolean` expression.

Remarks

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 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`	`True`	`True`
`True`	`False`	`False`
`False`	(not evaluated)	`False`

Note

In a Boolean comparison, the And operator always evaluates both expressions, which could include making procedure calls. The AndAlso Operator performs short-circuiting, which means that if expression1 is False, then expression2 is not evaluated.

Data Types

The AndAlso operator is defined only for the Boolean Data Type. Visual Basic converts each operand as necessary to Boolean before evaluating the expression. If you assign the result to a numeric type, Visual Basic converts it from Boolean to that type such that False becomes 0 and True becomes -1. For more information, see Boolean Type Conversions.

Overloading

The And Operator and the IsFalse Operator 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 And and IsFalse operators affects the behavior of the AndAlso operator. If your code uses AndAlso on a class or structure that overloads And and IsFalse, be sure you understand their redefined behavior. For more information, see Operator Procedures.

Example 1

The following example uses the AndAlso operator to perform a logical conjunction on two expressions. The result is a Boolean value that represents whether the entire conjoined expression is true. If the first expression is False, 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 AndAlso b > c
secondCheck = b > a AndAlso b > c
thirdCheck = a > b AndAlso c > b

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

Example 2

The following example shows a Function procedure that searches for a given value among the elements of an array. If the array is empty, or if the array length has been exceeded, the While statement does not test the array element against the search value.

Public Function findValue(ByVal arr() As Double, 
    ByVal searchValue As Double) As Double
    Dim i As Integer = 0
    While i <= UBound(arr) AndAlso arr(i) <> searchValue
        ' If i is greater than UBound(arr), searchValue is not checked.
        i += 1
    End While
    If i > UBound(arr) Then i = -1
    Return i
End Function

Jaa