Transcript Tut 7
Tutorial 7
Sub and Function Procedures
Tutorial 7: Sub and Function Procedures
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Creating Sub and
Function Procedures
Lesson A Objectives
After completing this lesson, you will be able to:
Explain the difference between a Sub procedure
and a Function procedure
Create a procedure that receives information
passed to it
Explain the difference between passing data by
value and by reference
Create a Function procedure
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Procedures
A procedure is a block of program code that
performs a specific task
Procedures in Visual Basic .NET can be either
Sub procedures or Function procedures
Function procedures return a value after
performing their assigned task
Sub procedures do not return a value
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Sub Procedures
Event procedures
Called by Visual Basic .NET in response to an
event
Every event procedure has at least two parameters
• sender – the object that raised the event
• e – information about the object
User-defined procedures
You must call explicitly
You can define parameters
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Event Procedures
Private Sub ExitButton_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) _
Handles ExitButton.Click
Parameters
Me.Close()
End Sub
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Including Parameters in
a User-Defined Sub Procedure
User-defined Sub procedures have both a procedure
header and procedure footer
Accessibility Sub ProcedureName([ParameterList])
‘ Statements in the procedure
End Sub
Private Sub CalculatePay(ByVal sngHours As Single, _
ByVal sngRate As Single, ByRef sngPay As Single)
‘ Calculate the pay from hours and rate of pay
sngPay = sngRate * sngHours
End Sub
Call CalculatePay(35.9, 95, sngPay)
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Passing Variables
Pass by value – make a copy of the data
Use ByVal before the parameter name
This is the default if you do not specify
Pass by reference – pass the address of the data
Use ByRef before the parameter name
Private Sub CalculatePay(ByVal sngHours As Single, _
ByVal sngRate As Single, ByRef sngPay As Single)
‘ Calculate the pay from hours and rate of pay
sngPay = sngRate * sngHours
End Sub
sngHours = 35.9: sngRate = 95.0
Call CalculatePay(sngHours, sngRate, sngPay)
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Function Procedures
A Function procedure, typically referred to as a
function, is a block of code that performs a
specific task
You can also create your own functions, referred
to as user-defined functions, in Visual Basic .NET
The Return statement alerts the computer that the
function has completed its task and ends the
function after returning the value of its
expression
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Function Procedures
Accessibility Function FunctionName([ParameterList]) As DataType
‘ Statements in the procedure
Return SomeData
End Function
Private Function CalculatePay(ByVal sngHours As Single, _
ByVal sngRate As Single) As Single
‘ Calculate the pay from hours and rate of pay
Return sngRate * sngHours
End Function
sngHours = 35.9: sngRate = 95.0
sngPay= CalculatePay(sngHours,sngRate)
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Using a List Box Control
Lesson B Objectives
After completing this lesson, you will be able to:
Add a list box to a form
Add items to a list box
Sort the contents of a list box
Select a list box item from code
Determine the selected item in a list box
Code a list box’s SelectedValueChanged event
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Adding a List Box to a Form
You can use a list box control to display a list of
choices from which the user can select zero
choices, one choice, or more than one choice
The number of choices the user is allowed to
select is controlled by the list box control’s
SelectionMode property
The Windows standard for list boxes is to display
a minimum of three selections and a maximum of
eight selections at a time
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Adding Items to a List Box
The items in a list box belong to a collection
called the Items collection
The first item in the Items collection appears
as the first item in the list box
The second item appears as the second item in
the list box, and so on
The first item in the Items has an index of zero
The second item has an index of one, and so
on
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Adding Items to a List Box
You use the Items collection’s Add method to
specify the items you want displayed in a list box
control
When you use the Add method to add an item to a
list box, the position of the item in the list
depends on the value stored in the list box’s
Sorted property
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The SelectedItem
and SelectedIndex Properties
A list box’s SelectItem property and its SelectedIndex
property can be used both to determine the item selected in
the list box and to select a list box item from code
The selected item is also called the default list box item
Should be either the most used selection
Or, if all of the selections are used fairly equally, the first
selection in the list
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Coding the GetFwtTax Function
The amount of federal withholding text (FWT) to
deduct from an employee’s weekly gross pay is
based on the employee’s filing status—either
single, (including head of household) or
married—and his or her weekly taxable wages
To calculate the federal withholding tax you need
to know the employee’s
Gross pay amount
Marital status
Number of withholding allowances
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Completing the CalculateButton
Click Event Procedure
Now that you have created the GetFwtTax
function, you can call the function from
the CalculateButton Click event procedure
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Clearing the Contents
of the Label Controls
The label controls also should be cleared
when the SelectedValueChanged event
occurs for one of the list boxes in the
interface
A list box’s SelectedValueChanged event
occurs each time a different value is
selected in the list box
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Completing the Payroll Application
Lesson C Objectives
After completing this lesson, you will be able to:
Add an existing form to a solution
Add a new module to a solution
Code the Sub Main Procedure
Create an instance of a form
Display a form object using the ShowDialog
method
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Adding an Existing Form to a Solution
The Copyright screen is to be the splash
screen for each custom application
created by the company
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Coding the Sub Main Procedure
Sub Main is a special procedure in Visual Basic
.NET, because it can be declared as the “starting
point” for an application
In other words, you can tell the computer to
process the Sub Main procedure automatically
when an application is started
You enter the Sub Main procedure in a module,
which is a file that contains code that is not
associated with any specific object in the
interface
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Creating an Instance of a Form
A class definition specifies (or defines) the attributes and
behaviors of an object
When an application is started, Visual Basic .NET
automatically processes the code contained in one object:
the Startup object
Similarly, if the PayrollForm is specified as the Startup
object, Visual Basic .NET automatically processes the code
contained in the PayrollForm class definition
When the Sub Main procedure is the Startup object, as it is
in this case, neither the CopyrightForm class definition nor
the PayrollForm class definition will be processed
automatically
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Using a Form Object’s
Show Dialog Method
The form object’s Show dialog method
allows you to display a form object on the
screen
The syntax of the ShowDialog method is
form.ShowDialog(), where form is the
name of the object variable that contains
the form object’s address
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