C# Programming: From Problem Analysis to Program Design1 9 Programming Based on Events.

C# Programming: From Problem Analysis to Program Design 1

9 Programming Based on Events


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


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


Delegates• Delegates store references (addresses) to methods,

as opposed to storing actual data

– Delegates form the foundation for events in C#

• Declaration for a delegate looks more like a method declaration than a class definition

– Except, delegate declaration has no body

– Declaration begins with the keyword delegate

• Declaration ends with a parenthesized list of parameters


Delegates (continued)

• Delegate declaration exampledelegate string ReturnsSimpleString( );

• Delegate signature– Identifies what types of methods the delegate represents

– The return type of a delegate becomes part of its identifying signature

• Above Example represents methods that return a string and require no argument static string EndStatement( )

static string ToString( )

static string ReturnSaying( )


Delegates (continued)

• Associate delegate with method(s) by creating delegate instance(s)

– Example

ReturnsSimpleString saying3 = new

ReturnsSimpleString(EndStatement);

• Constructor for delegate of the delegate class always takes just parameter

– Name of a method for the constructor to reference


Delegates (continued)• Delegate identifier references the method sent as

argument to constructor

– Any use of delegate identifier now calls the method

• Methods are said to be wrapped by the delegate

– Delegate can wrap more than one method, called a multicast delegate

• += and -= operators are used to add/remove methods to/ from the delegate chain or invocation list

• Multicast delegates must have a return type of void


Relationship of Delegates to Events• Delegates are used for event-driven application

– Delegate acts as intermediary between objects that are raising or triggering an event, and the object that captures the event and responds to it.

• During compilation, the method or methods that will be called are not determined

• Events as special forms of delegates

– Place a reference to event-handler methods inside a delegate

– Once reference is made, or event is registered, delegate is used to call event-handler method when an event like a button click is fired


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


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


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


Adding a ListBox Control Object

Add ListBox control, then

click on Items property

(Collection) to type entries

Figure 9-2 String Collection Editor


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


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 methodprivate void listBox1_SelectedIndexChanged

(object sender, System.EventArgs e)

{

}


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



Figure 9-3 SelectedIndexChanged event fired


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;



Figure 9-4 Multiple selections within a ListBox object


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);

}


Figure 9-5 Add( ) method executed inside the buttonClick event

ListBoxExample



ListBox Control Properties


ListBox Control Methods


Note that ListBox control inherits members from Control class

ListBox Control Methods (continued)


ComboBox Controls

Extra TextBox object with ComboBox – User selects from list or types new value

Figure 9-6 ComboBox and ListBox objects


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


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


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


Programming Event Handlers (continued)

Figure 9-8 KeyPress and SelectedIndexChanged events fired

KeyPress( ) event-

handler method

fired with each

keystroke


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


MenuStrip Controls (continued)

Drag MenuStrip control to form,

then click here to display Menu

structure

Figure 9-9 First step to creating a menu


MenuStrip Control Objects • Ampersand (&) is typed between the F and o for the

Format option to make Alt+o shortcut for Format

Figure 9-10 Creating a shortcut for a menu item


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


MenuStrip Control Objects (continued)

Figure 9-12 Setting the Property for the ToolTip control

Set the text to be displayedwhen the cursor is rested on top of the control


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


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


Adding Predefined Standard Windows Dialog Boxes – Color

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

Retrieves the current ForeColor property setting

for the Label object

Checks to see if Cancel button

clicked Set to

selection made


Adding Predefined Standard Windows Dialog Boxes – Color (continued)

Figure 9-14 Color dialog box menu option


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


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


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


Wiring One Event Handler to Multiple Objects (continued)

Figure 9-16 Wiring the event-handler method


CheckBox Object

Figure 9-17 ComputeCost_CheckedChanged( ) method raised


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


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


RadioButton Objects (continued)

Figure 9-18 GroupBox and RadioButton objects added



• 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


RadioButton Objects (continued)• Register ComputeCost_CheckedChanged( )

method

Figure 9-19 Wired Click event



• ComputeCost_CheckedChanged( ) methodif (this.radBeginner.Checked){ cost +=10; this.lblMsg.Text = "Beginner “ + “-- Extra $10 charge";}else// more statements


ComputeCost_CheckChanged( ) and Click( ) Events Raised

Figure 9-20 ComputeCost_CheckedChanged( ) and Click( ) events raised


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


TabControl Controls (continued)

Figure 9-21 Tabbed controlled application



Figure 9-22 TabControl object stretched to fill form



• TabPage property enables you to format individual tabs

• Clicking the ellipsis beside the Collection value displays the TabPage Collection Editor


Chapter Summary• Event-handling procedures

– Registering an event

• ListBox control for multiple selections

• ComboBox versus ListBox objects


Chapter Summary (continued)• Adding controls to save space

– MenuStrip controls

– TabControl

• Use of GroupBox controls

• RadioButton versus CheckBox objects

C# Programming: From Problem Analysis to Program Design1 9 Programming Based on Events.

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