Microsoft Visual Basic 2010 Step by Step eBook

162 Part II Programming Fundamentals

in the program and take a course of action based on the result. In its simplest form,

an If... Then decision structure is written on a single line:

If condition Then statement

where condition is a conditional expression, and statement is a valid Visual Basic program

statement. For example:

If Score >= 20 Then Label1.Text = "You win!"

is an If... Then decision structure that uses the conditional expression:

Score >= 20

to determine whether the program should set the Text property of the Label1 object to “You

win!” If the Score variable contains a value that’s greater than or equal to 20, Visual Basic

sets the Text property; otherwise, it skips the assignment statement and executes the next

line in the event procedure. This sort of comparison always results in a True or False value.

A conditional expression never results in a value of maybe.

Testing Several Conditions in an If... Then

Decision Structure

Visual Basic also supports an If... Then decision structure that you can use to include several

conditional expressions. This block of statements can be several lines long and contains the

important keywords ElseIf, Else, and End If:

If condition1 Then

statements executed if condition1 is True

ElseIf condition2 Then

statements executed if condition2 is True

[Additional ElseIf conditions and statements can be placed here]

Else

statements executed if none of the conditions is True

End If

In this structure, condition1 is evaluated first. If this conditional expression is True, the block

of statements below it is executed, one statement at a time. (You can include one or more

program statements .) If the first condition isn’t True, the second conditional expression

(condition2) is evaluated. If the second condition is True, the second block of statements

is executed. (You can add additional ElseIf conditions and statements if you have more

conditions to evaluate .) If none of the conditional expressions is True, the statements below

the Else keyword are executed. Finally, the whole structure is closed by the End If keywords.

The following code shows how a multiple-line If... Then structure could be used to

determine the amount of tax due in a hypothetical progressive tax return. (The income

and percentage numbers are from the projected U .S. Internal Revenue Service 2010 Tax Rate

Schedule for single filing status .)