Programming Based on Events

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9

• Programming Based on Events

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Chapter Objectives

• Explore event-handling procedures in C by
  writing and registering event-handler methods
• Create applications that use the ListBox control
  object to enable multiple selections from a
  single control
• Contrast ComboBox to ListBox objects by adding
  both types of controls to an application

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Chapter Objectives (continued)

• Add Menu and TabControl control options to Window
  forms and program their event-handler methods
• Wire multiple RadioButton and CheckBox object
  events to a single event-handler method

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Event Handling in C

• Form Designer in Visual Studio did much of the
  work for you
• Double-clicked on a Button control object during
  design
• 1) Click event is registered as being of interest
• 2) An event-handler method heading is generated
• Two steps form event wiring process
• Wire an event associate (identify) a method to
  handle its event

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Event Handling in C (continued)

• Code associates the methods with an event
• this.button1.Click new System.EventHandler(this
  .button1_Click)
• this.button2.Click new System.EventHandler(this
  .button2_Click)
• button1.Click and button2.Click are methods
• Keyword this is added to all code generated by
  Visual Studio to indicate the current instance of
  a class

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ListBox Control Objects

• Displays list of items for single or multiple
  selections
• Scroll bar is automatically added when total
  number of items exceeds the number that can be
  displayed
• Can add or remove items at design time or
  dynamically at run time
• Includes number of properties and events
• The Items property used to set initial values
• Click on (Collections) to add items

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Adding a ListBox Control Object
Add ListBox control, then click on Items property
(Collection) to type entries
Figure 9-2 String Collection Editor
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ListBox Control Objects (continued)

• Name property
• Useful to set for program statements
• Sorted property
• Set to true to avoid having to type values in
  sorted order
• Register an event for the ListBox
• Might want to know when the item selection
  changes
• Double-clicking on any control registers its
  default event for the control
• SelectedIndexChanged default event for ListBox

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ListBox Control Objects (continued)

• Register its event with the System.EventHandler
• this.lstBoxEvents.SelectedIndexChanged new
  System.EventHandler
  (this.listBox1_SelectedIndexChanged)
• Visual Studio adds event-handler method
• private void listBox1_SelectedIndexChanged
• (object
  sender, System.EventArgs e)

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ListBox Control Objects (continued)

• To retrieve string data from ListBox use Text
  property
• this.txtBoxResult.Text this.lstBoxEvents.Text
• Place in method body
• When event fires, selection retrieved and stored
  in TextBox object

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ListBox Control Objects (continued)
Figure 9-3 SelectedIndexChanged event fired
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Multiple Selections with a ListBox

• SelectionMode Property has values of MultiSimple,
  MultiExtended, None, and One
• MultiSimple use the spacebar and click the mouse
• MultiExtended can also use Ctrl key, Shift key,
  and arrow keys
• foreach(string activity in
  lstBoxEvents.SelectedItems)
• result activity " "
• this.txtBoxResult.Text result

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ListBox Control Objects (continued)
Figure 9-4 Multiple selections within a ListBox
object
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ListBox Control Objects (continued)

• SelectedItem and SelectedItems return objects
• Store numbers in the ListBox, once retrieved as
  objects, cast the object into an int or double
  for processing
• Adding items to a ListBox at run time by using
  Add( ) method with the Items property
• lstBoxEvents.Items.Add("string value to add")
• private void btnNew_Click(object sender,
  System.EventArgs e)
• lstBoxEvents.Items.Add(txtBoxNewAct.Text)

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ListBoxExample
Figure 9-5 Add( ) method executed inside the
buttonClick event
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ListBox Control Properties
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ListBox Control Methods
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ListBox Control Methods (continued)
Note that ListBox control inherits members from
Control class
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ComboBox Controls
Extra TextBox object with ComboBox User selects
from list or types new value
Figure 9-6 ComboBox and ListBox objects
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ComboBox Controls (continued)
Top line left blank in ComboBox when
DropDownStyle property is set to DropDown
(default setting)
Figure 9-7 ComboBox list of choices
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Handling ComboBox Events

• ComboBox only allows a single selection to be
  made
• Default event-handler method SelectedIndexChanged
  ( )
• Same as ListBox control object
• Could register KeyPress( ) event-handler method
• BUT, event is fired with each and EVERY keystroke

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Programming Event Handlers

• Since ListBox object allows multiple selections,
  Text property cannot be used
• Text ONLY gets the first one selected
• Use the SelectedItems, SelectedIndices, or Items
  to retrieve a collection of items selected
• Zero-based structures
• Access them as you would access an element from
  an array
• SelectedIndices is a collection of indexes

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Programming Event Handlers (continued)
KeyPress( ) event-handler method fired with each
keystroke
Figure 9-8 KeyPress and SelectedIndexChanged
events fired
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MenuStrip Controls

• Offers advantage of taking up minimal space
• Drag and drop MenuStrip object from toolbox to
  your form
• Icon representing MenuStrip placed in Component
  Tray
• Select MenuStrip object to set its properties
• To add the text for a menu option, select the
  MenuStrip icon and then click in the upper-left
  corner of the form

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MenuStrip Controls (continued)
Drag MenuStrip control to form, then click here
to display Menu structure
Figure 9-9 First step to creating a menu
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MenuStrip Control Objects

• Ampersand () is typed between the F and o for
  the Format option to make Alto shortcut for
  Format

Figure 9-10 Creating a shortcut for a menu item
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MenuStrip Control Objects (continued)

• To create separators, right-click on the text
  label (below the needed separator)
• Select Insert Separator

Figure 9-11 Adding a separator
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MenuStrip Control Objects (continued)
Set the text to be displayed when the cursor is
rested on top of the control
Figure 9-12 Setting the Property for the ToolTip
control
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Wire Methods to Menu Option Event

• Set the Name property for each menu option
• Do this first, then wire the event
• Click events are registered by double-clicking on
  the Menu option
• When the menu option is clicked, the event
  triggers, happens, or is fired

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Adding Predefined Standard Windows Dialog Boxes

• Included as part of .NET
• Dialog boxes that look like standard Windows
  dialog boxes
• File Open, File Save, File Print, and File Print
  Preview
• Format Font
• Format Color dialogs

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Adding Predefined Standard Windows Dialog Boxes
Color
Retrieves the current ForeColor property setting
for the Label object

• private void menuColor_Click(object sender,
• System.EventArgs e)
• colorDialog1.Color lblOutput.ForeColor
• if (colorDialog1.ShowDialog( ) !
  DialogResult.Cancel )
• lblOutput.ForeColor colorDialog1.Color

Checks to see if Cancel button clicked
Set to selection made
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Adding Predefined Standard Windows Dialog Boxes
Color (continued)
Figure 9-14 Color dialog box menu option
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Adding Predefined Standard Windows Dialog Boxes
Font

• private void menuFont_Click (object sender,
  System.EventArgs e)
• fontDialog1.Font
• lblOutput.Font
• if (fontDialog1.ShowDialog( )
• ! DialogResult.Cancel )
• lblOutput.Font
• fontDialog1.Font

Figure 9-15 Font dialog box menu option
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CheckBox Objects

• Appear as small boxes
• Allow users to make a yes/no or true/false
  selection
• Checked property set to either true or false
  depending on whether a check mark appears or not
• Default false value
• CheckChanged( ) default event-handler method
• Fired when CheckBox object states change
• Can wire one event handler to multiple objects

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Wiring One Event Handler to Multiple Objects

• Using Properties window, click on the Events Icon
  
• Click the down arrow associated with that event
• Select method to handle the event
• Follow the same steps for other objects

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Wiring One Event Handler to Multiple Objects
(continued)
Figure 9-16 Wiring the event-handler method
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CheckBox Object
Figure 9-17 ComputeCost_CheckedChanged( ) method
raised
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GroupBox Objects

• CheckBox objects may be grouped together for
  visual appearance
• Can move or set properties that impact the entire
  group
• A GroupBox control should be placed on the form
  before you add objects
• GroupBox control adds functionality to
  RadioButton objects
• Allow only one selection

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RadioButton Objects

• Appear as small circles
• Give users a choice between two or more options
• Not appropriate to select more than one CheckBox
  object with RadioButton objects
• Group RadioButton objects by placing them on a
  Panel or GroupBox control
• Setting the Text property for the GroupBox adds a
  labeled heading over the group

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RadioButton Objects (continued)
Figure 9-18 GroupBox and RadioButton objects added
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RadioButton Objects (continued)

• Turn selection on
• this.radInterm.Checked true
• Raise a number of events, including Click( ) and
  CheckedChanged( ) events
• Wire the event-handler methods for RadioButton
  objects, just like CheckBox

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RadioButton Objects (continued)

• Register ComputeCost_CheckedChanged( ) method

Figure 9-19 Wired Click event
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RadioButton Objects (continued)

• ComputeCost_CheckedChanged( ) method
• if (this.radBeginner.Checked)
• cost 10
• this.lblMsg.Text
• "Beginner
• -- Extra 10 charge"
• else
• // more statements

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ComputeCost_CheckChanged( ) and Click( ) Events
Raised
Figure 9-20 ComputeCost_CheckedChanged( ) and
Click( ) events raised
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TabControl Controls

• Sometime an application requires too many
  controls for a single screen
• TabControl object displays multiple tabs, like
  dividers in a notebook
• Each separate tab can be clicked to display other
  options
• Add a TabControl object to the page by dragging
  the control from the Container section of the
  Toolbox

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TabControl Controls (continued)
Figure 9-21 Tabbed controlled application
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TabControl Controls (continued)
Figure 9-22 TabControl object stretched to fill
form
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TabControl Controls (continued)

• TabPage property enables you to format individual
  tabs
• Clicking the ellipsis beside the Collection value
  displays the TabPage Collection Editor

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Chapter Summary

• Event-handling procedures
• Registering an event
• ListBox control for multiple selections
• ComboBox versus ListBox objects

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Chapter Summary (continued)

• Adding controls to save space
• MenuStrip controls
• TabControl
• Use of GroupBox controls
• RadioButton versus CheckBox objects