Microsoft Visual Basic 2010 Step by Step eBook

262 Part II Programming Fundamentals

Writing Sub Procedures

A Sub procedure is similar to a Function procedure, except that a Sub procedure doesn’t

return a value associated with its name. Sub procedures are typically used to get input

from the user, display or print information, or manipulate several properties associated with

a condition. Sub procedures can also be used to process and update variables received in

an argument list during a procedure call and pass back one or more of these values to the

calling program.

Sub Procedure Syntax

The basic syntax for a Sub procedure is:

Sub ProcedureName([arguments])

procedure statements

End Sub

The following syntax items are important:

n ProcedureName is the name of the Sub procedure you’re creating.

n arguments is a list of optional arguments (separated by commas if there’s more than

one) to be used in the Sub procedure. Each argument should also be declared as

a specific type. (Visual Studio adds the ByVal keyword by default to each argument,

indicating that a copy of the data is passed to the function through this argument but

that any changes to the arguments won’t be returned to the calling routine .)

n procedure statements is a block of statements that accomplishes the work of the

procedure.

In the Sub procedure call, the number and type of arguments sent to the procedure must

match the number and type of arguments in the Sub procedure declaration, and the entire

group must be enclosed in parentheses. If variables passed to a Sub procedure are modified

during the procedure, the updated variables aren’t passed back to the program unless the

procedure defined the arguments by using the ByRef keyword. Sub procedures declared in

a module are public by default, so they can be called by any event procedure in a project.

Important All calls to a Sub procedure must include parentheses after the procedure

name. A set of empty parentheses is required even if no arguments are being passed to the

procedure.

For example, the following Sub procedure receives a string argument representing a person’s

name and uses a text box to wish that person happy birthday. If this Sub procedure is

declared in a module, it can be called from any event procedure in the program.