Microsoft Visual Basic 2010 Step by Step eBook

170 Part II Programming Fundamentals

of an If statement makes logical sense—why should Visual Basic continue to evaluate the If

statement if both conditions cannot be True?

The OrElse operator works in a similar fashion. Consider an If statement that has two

conditions that are connected by an OrElse operator. For the statements of the If structure

to be executed, at least one condition must evaluate to True. If the first condition evaluates

to True, Visual Basic begins to execute the statements in the If structure immediately, without

testing the second condition.

Here’s an example of the short-circuit situation in Visual Basic, a simple routine that uses

an If statement and an AndAlso operator to test two conditions and display the message

“Inside If” if both conditions are True:

Dim Number As Integer = 0

If Number = 1 AndAlso MsgBox("Second condition test") Then

MsgBox("Inside If")

Else

MsgBox("Inside Else")

End If

The MsgBox function itself is used as the second conditional test, which is somewhat unusual,

but the strange syntax is completely valid and gives us a perfect opportunity to see how

short-circuiting works up close. The text “Second condition test” appears in a message

box only if the Number variable is set to 1; otherwise, the AndAlso operator short-circuits

the If statement, and the second condition isn’t evaluated. If you actually try this code,

remember that it’s for demonstration purposes only—you wouldn’t want to use MsgBox

with this syntax as a test because it doesn’t really test anything. But by changing the Number

variable from 0 to 1 and back, you can get a good idea of how the AndAlso statement and

short-circuiting work.

Here’s a second example of how short-circuiting functions in Visual Basic when two

conditions are evaluated using the AndAlso operator. This time, a more complex conditional

test (7 / HumanAge <= 1) is used after the AndAlso operator to determine what some

people call the “dog age” of a person:

Dim HumanAge As Integer

HumanAge = 7

'One year for a dog is seven years for a human

If HumanAge <> 0 AndAlso 7 / HumanAge <= 1 Then

MsgBox("You are at least one dog year old")

Else

MsgBox("You are less than one dog year old")

End If

As part of a larger program that determines the so-called dog age of a person by dividing

his or her current age by 7, this bare-bones routine tries to determine whether the value

in the HumanAge integer variable is at least 7. (If you haven’t heard the concept of “dog

age” before, bear with me—following this logic, a 28-year-old person would be four dog