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Performs short-circuiting logical conjunction on two expressions.
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
One use of the AndAlso operator is to test for the existence of an object instance before attempting to access one of its members. The following code line illustrates this.
If newObject AndAlso newObject.initFinished Then
The access to the initFinished property in the preceding code line could throw a NullReferenceException exception if the newObject variable did not have an object instance assigned to it. However, the AndAlso operator causes the compiler to bypass the access to initFinished if newObject is Nothing, because Nothing evaluates to False.
The AndAlso operator is defined only for the Boolean Data Type (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. For example, 5 AndAlso 12 results in –1 when converted to Integer.
The And Operator (Visual Basic) 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.
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.
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.
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