1. Programming Based on Events10
C# Programming: From Problem Analysis to Program Design
3rd Edition
C# Programming: From Problem Analysis to Program Design

2. Chapter Objectives
Define, create, and use delegates and examine their relationship to events
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
C# Programming: From Problem Analysis to Program Design

3. 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
Design and create a Windows Presentation Foundation (WPF) application
Work through a programming example that illustrates the chapter’s concepts
C# Programming: From Problem Analysis to Program Design

4. 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
Unlike a method, the return type of a delegate becomes part of its identifying signature
C# Programming: From Problem Analysis to Program Design

5. Defining Delegates Delegate declaration example Delegate signature
delegate string ReturnsSimpleString( );
Delegate signature
Identifies what types of methods the delegate represents
Above Example represents methods that return a string and require no argument
static string EndStatement( )
static string ToString( )
static string ReturnSaying( )
C# Programming: From Problem Analysis to Program Design

6. Creating Delegate Instances
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
C# Programming: From Problem Analysis to Program Design

7. Using Delegates
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
C# Programming: From Problem Analysis to Program Design

8. 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
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
C# Programming: From Problem Analysis to Program Design

9. 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
C# Programming: From Problem Analysis to Program Design

10. Event-Handler Methods
Code associates the methods with a delegate
this.button1.Click += new System.EventHandler(this.button1_Click);
this.button2.Click += new System.EventHandler(this.button2_Click);
System.EventHandler is a delegate type
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
C# Programming: From Problem Analysis to Program Design

11. 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 is used to set initial values
Click on (Collections) to add items
C# Programming: From Problem Analysis to Program Design

12. Adding a ListBox Control Object
Add ListBox control, then click on Items property (Collection) to type entries
Figure 10-2 String Collection Editor
C# Programming: From Problem Analysis to Program Design

13. Registering a ListBox Event
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
C# Programming: From Problem Analysis to Program Design

14. ListBox Event Handlers
Register its event with the System.EventHandler delegate
this.lstBoxEvents.SelectedIndexChanged += new System.EventHandler (this.listBox1_SelectedIndexChanged);
Visual Studio adds event-handler method
private void listBox1_SelectedIndexChanged
(object sender, System.EventArgs e) { }
C# Programming: From Problem Analysis to Program Design

15. ListBox Control Objects
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
C# Programming: From Problem Analysis to Program Design

16. ListBox Control Objects (continued)
Figure 10-3 SelectedIndexChanged event fired
C# Programming: From Problem Analysis to Program Design

17. 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;
C# Programming: From Problem Analysis to Program Design

18. ListBox Control Objects (continued)
Figure 10-4 Multiple selections within a ListBox object
C# Programming: From Problem Analysis to Program Design

19. 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
C# Programming: From Problem Analysis to Program Design

20. Adding Items to a ListBox
Add items at run time 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); }
C# Programming: From Problem Analysis to Program Design

21. ListBoxExample
Figure 10-5 Add( ) method executed inside the buttonClick event
C# Programming: From Problem Analysis to Program Design

22. C# Programming: From Problem Analysis to Program Design

23. ListBox Control Properties
C# Programming: From Problem Analysis to Program Design

24. C# Programming: From Problem Analysis to Program Design

25. ComboBox Objects
In many cases, ListBox controls and ComboBox controls can be used interchangeably
ListBox control is usually used when you have all the choices available at design time
ComboBox facilitates displaying a list of suggested choices with an added feature
ComboBox objects contain their own text box field as part of the object
C# Programming: From Problem Analysis to Program Design

26. Adding ComboBox Controls
Extra TextBox object with ComboBox – User selects from list or types new value
Figure 10-6 ComboBox and ListBox objects
C# Programming: From Problem Analysis to Program Design

27. Adding ComboBox Controls (continued)
Top line left blank in ComboBox when DropDownStyle property is set to DropDown (default setting)
Figure 10-7 ComboBox list of choices
C# Programming: From Problem Analysis to Program Design

28. 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
C# Programming: From Problem Analysis to Program Design

29. Programming Event Handlers
Since ListBox object allows multiple selections, Text property cannot be used
Text ONLY gets the first one selected
Use 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
C# Programming: From Problem Analysis to Program Design

30. Programming Event Handlers (continued)
KeyPress( ) event-handler method fired with each keystroke
Figure 10-8 KeyPress and SelectedIndexChanged events fired
C# Programming: From Problem Analysis to Program Design