2: Controlling Your Programs in VS .NET

Maker GTIN - 12 in VS .NET 2: Controlling Your Programs

Setting conditions with the If statement Zeroing in on the Select Case statement Reaching a particular value with the For statement Setting a time limit with the While statement Specifying tasks with the Do statement

nly the most trivial of programs execute their tasks one at a time, in the exact sequence in which the statements are listed in the program Life, even in the world of programming, just isn t that simple Most programs require more control over the sequence in which statements are executed For example, you may need to skip over some statements based on the results of a condition test Or, you may need to create a loop of statements that repeats itself a given number of times or until a certain condition is met This chapter describes the VBNET statements that let you control the flow of execution for your ASPNET programs

The Robert Frost poem that begins, Two roads diverged in a yellow wood is an apt description of how the If statement works The program is rolling along, executing one statement after another, until it comes to an If statement The If statement represents a fork in the road; a choice must be made about which path to take Many programs have to make such either/or decisions as they execute For example, a program that writes data entered by the user into a database has to make a determination first: Did the user actually enter some data If so, the program writes the data to the database If not, the program displays an error message saying something along the lines of, Yo! You didn t enter anything! To process this decision, the program uses an If statement

VB s If statement is remarkably flexible, with several formats to choose from All these forms involve three basic parts: A condition test, in which a conditional expression is evaluated to yield a value of True or False (There s more about conditional expressions later in this chapter) A Then part, which supplies one or more statements that execute only if the result of the condition test is True An Else part, which supplies one or more statements that execute only if the result of the condition test is False For example, consider these lines:

If txtLastNameText <> Then Add the last name to the database Else Display an error message End If

Here the If statement begins with a condition test that checks to see whether the user entered anything in the txtLastName text box If so, that data is written to the database; if not, an error message is displayed The If statement takes various useful forms, with varying degrees of complexity The following sections dig deeper into the basics of using them

The most basic form of the If statement looks like this:

Each component of the If statement must fall on a separate line, as shown in the preceding structure In other words, you shouldn t place the statements on the same line as the Then or Else keywords Here s an example of a typical If statement:

In this example, a variable named CommissionRate is set to 005 if Sales Total is greater than $10,000

Here s an example that executes more than one statement if the condition test is True:

Dim CommissionRate As Decimal If SalesTotal > 10000D Then CommissionRate = 005D Commission = SalesTotal * CommissionRate End If

VBNET also allows you to use a single-line form of the If statement, which looks like this:

If condition Then statement [Else statement]

To use this form of the If statement, the condition, Then clause, and Else clause (if any) must all be coded on the same line Here s an example:

If x > 0 Then ResponseWrite( X is & x)

The preceding example displays the message X is n, where n is the value of x But the message appears only if x is greater than zero You can include more than one statement in the Then part by separating the statements with colons But if more than one statement is required, or if an Else part is required, I suggest you use the basic multiline If form instead of the single-line form