CS0004: Introduction to
Programming
Events
Review

Event Procedure


A set of instructions to be executed when a certain event
happens.
Many event-driven programs can be divided into three
important sections or layers:

Presentation Layer


Logic Layer

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Anything the user sees and how the user interacts with a program.
How the program acts when it receives events.
Data Access Layer

Allows simple access to persistent storage (like a database). We
probably won’t be doing this in this course.
Review

When you run your Visual Basic program the following
things happen:
1.
2.
3.
4.
Your program monitors the controls in the window to detect
any event that a control can recognize (mouse movements,
clicks, keystrokes, etc.)
When your program detects an event, it examines the code
to see if you’ve written an even procedure for it.
If you have written an event procedure, the instructions in
the procedure are executed and it goes back to step 1.
If you have not written an event procedure, it ignores the
event and goes back to step 1.
Review

The naming (value of the name property) of controls
should indicate what it is and what it does.
Object
Prefix
Example
Form
frm
frmPayroll
Button
btn
btnComputeTotal
Label
lbl
lblAddress
Text Box
txt
txtCity
List Box
lst
lstOutput
Events



Now that we have an interface, we can have our controls act
on events.
For now, we will just change the properties of controls with
our events
The basic form of setting a property of a control:
controlName.property = setting



controlName – the name of the control whose property
you want to change (ex. txtName)
property – the property you want to change (ex.
ForeColor)
setting – value for which you want to set the property
Events
Examples:
txtBox.Text = “Hello”
 Displays “Hello” in the text box
btnButton.Visible = True
 Makes the button visible
txtBox.ForeColor = Color.Red
 Makes the text color in the text box red
frmMain.Text = “My Form”
 Will NOT work in the code for frmMain. You must use the
word Me (because the form is referring to itself)
Me.Text = “My Form”
 These lines of code (statements) are called Assignment
Statements.

Event Procedures



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To give the controls function when events occur, we need
to write our code in event procedures.
Note: I will often call event procedures, “event handlers”.
To view the code right click on the form and select “View
Code”
To make an event procedure you can:
1.
2.

Double click on the control (creates the default event
procedure for the control)
Click on the control then click the “Events” button at the
top of the Properties Window
This will create the code for the event procedure.
Event Procedures

An event procedure looks something like this:
Private Sub btnButton_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles btnButton.Click
‘Code goes here
End Sub


Don’t worry about all the details of this code!
There’s only two parts you need to worry about:
1.
2.

btnButton_Click is the name of the event procedure
btnButton.Click tells the procedure to listen for the click
event on the button btnButton.
The most of the rest are keywords used to describe the
procedure


Keywords are words that have a particular meaning in VB
These are reserved, meaning they can’t be used for other purposes
(this will be clearer when we talk about variables)
Event Procedures

A single event handler can listen for multiple different
events.
Private Sub btnButton_Click(…) Handles
btnButton1.Click, btnButton2.Click
‘Code goes here
End Sub

So the code inside of the procedure will be executed if either
btnButton1 or btnButton2 is clicked.
IntelliSense


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
When you start typing code in Visual Studio, a system
called IntelliSense will attempt to assist you by giving you
options to complete your code.
This limits the amount of code you have to memorize.
When an item is highlighted in the drop-down supplied by
IntelliSense, a tool tip is supplied next to it.
You can select an item in the menu by clicking on it,
pushing enter, or tab.
If the IntelliSense drop-down gets in the way, you can hold
down the control key to make it transparent.
Default Events


When double clicking on a control, you will get the event
procedure for the control’s default event.
The some default events are as follows:
Control
Default Event
form
Load
button
Click
label
Click
list box
SelectedIndexChanged
text box
TextChanged
Some Events



Click – Useful for buttons.
DoubleClick – Not available for buttons, but can be used for labels, text boxes, etc.
Focus



Keys – Useful for text boxes.

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KeyDown – Event raised when a key is first pressed
KeyUp – Event raised when a key is first released
Key Pressed – Event raised when the control has focus and a key is pressed and released.
Mouse


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Enter – Event raised when control first receives focus
Leave – Event raised when control first loses focus
MouseDown – Event raised when the a mouse button is first pressed while over a control
MouseUp - Event raised when the a mouse button is first released while over a control
MouseEnter – Event raised when the mouse enters a visible part of the control
MouseLeave – Event raised when the mouse leaves a visible part of the control
MouseHover – Event raised when a mouse remains stationary over a control
Property Changed – When the programmer or the user changes a property of a
control an event is raised

TextChanged – Event raised when the value of the “Text” property is changed
Opening a Project

To open a saved VB project:
1.
2.
3.
4.

Also, if you successfully debug you program, it will create an
executable file inside of your Debug directory. To get to it:
1.
2.
3.
4.

Click on “Project/Solution” from the “File” menu under “Open”.
Navigate to where you saved your project.
Double Click on the folder that was created by Visual Studio when
you saved (it should have the same name that you named you project)
Double click on the “.sln” file (it should have the same name that you
named you project).
Double Click on the folder in the same folder as your “.sln” file.
Double Click on the “bin” folder.
Double Click on the “Debug” folder.
It is the “.exe” file in the “Debug” folder.
An executable file is a file for the specific purpose of running your
program.