1. INF230 Basics in C# Programming
AUBG, COS dept
Lecture 41
Event Handling
Reference: Doyle, chap 10

2. Lecture Contents:
Delegates and their relationship to events

3. Chapter 10 Programming Based on Events C# Programming:
From Problem Analysis to Program Design
4th Edition

4. Chapter 10
Programming Based on Events OR
Understanding Delegates

5. Recall data types Value types contain direct values Reference types
contain references to (address of) data
Examples:
SmallObject a = new SmallObject(713);
SparePart b = new SparePart(“200D”,45,7.97);
int[] c = new int[20];

6. New: delegates Special Reference types
contain references to (address of) a method or methods but not address of data
Examples:
Reference to method with signature: string return data type and no parameters
Reference to method with signature: long return data type, and one int parameter

7. Delegates
Delegates are special types of .NET classes whose instances store references to (addresses of) methods as opposed to storing actual data.
By tradition, you pass data into method through the use of parameters.
Delegates enable you to pass methods as parameters (as opposed to data) into other methods
Delegate signature:
Identifies what types of methods the delegate represents
For events, it is the address of which method to invoke when an event is fired
C# Programming: From Problem Analysis to Program Design

8. Understanding Delegates
Defining Delegates
Creating Delegate Instances
Using Delegates

9. Delegates (continued)
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 followed by semicolon
Unlike a method, the return type of a delegate becomes part of its identifying signature
Recall signature for method includes only the name and the number and type of parameters – not the return type
C# Programming: From Problem Analysis to Program Design

10. Example 1 Special Reference types
contain reference to (address of ) a method or methods but not data
Example:
Delegate as Reference to method with signature: string return data type and no parameters

11. Defining Delegates Delegate declaration
delegate string ReturnsSimpleString( );
Above example represents methods that return a string and require no argument
Given the above signature, any method that has zero parameters and returns a string, can be referenced by the ReturnsSimpleString delegate, including three below
static string AHeading( )
static string ToString( )
static string EndStatement( )
C# Programming: From Problem Analysis to Program Design

12. Defining Delegates
When you define a delegate type, you identify what types of methods the delegate represents.
Think of the delegate as a way of defining or naming a method signature
C# Programming: From Problem Analysis to Program Design

13. Understanding Delegates
Defining Delegates
Creating Delegate Instances
Using Delegates

14. Creating Delegate Instances
Associate delegate with method(s) by creating delegate instance(s)
ReturnsSimpleString saying3 = new ReturnsSimpleString(EndStatement);
Constructor for delegate of the delegate class always takes just one parameter (method name)
Name of a method for the constructor to reference
When you write a method name without parentheses (as above), you are referencing the address of the method.
C# Programming: From Problem Analysis to Program Design

15. Understanding Delegates
Defining Delegates
Creating Delegate Instances
Using Delegates

16. Using Delegates
Delegate identifier references the method sent as argument to constructor
Any use of delegate identifier now calls that 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

17. Using Delegates (continued)
delegate string ReturnsSimpleString( );
static void Main ( ) {
int age = 18;
ReturnsSimpleString saying1 = new ReturnsSimpleString(AHeading);
ReturnsSimpleString saying2;
saying2 = new ReturnsSimpleString((age + 10).ToString);
ReturnsSimpleString saying3= new ReturnsSimpleString(EndStatement);
MessageBox.Show(saying1( ) + saying2( ) + saying3( )); }
C# Programming: From Problem Analysis to Program Design

18. Using Delegates (continued)
These methods were associated (referenced) by the ReturnsSimpleString delegate
// Method that returns a string.
static string AHeading() {
return "Your age will be "; }
static string EndStatement( )
return " in 10 years.";
C# Programming: From Problem Analysis to Program Design

19. Using Delegates (continued)
Figure 9-1 Windows-based form
C# Programming: From Problem Analysis to Program Design

20. Example 1
Type, compile and run the example 1 application

21. Example 2 Special Reference types
contain reference(address) of method or methods but not data
Example:
Delegate as Reference to method with signature: long return data type, and one int parameter

22. Example 2 List of methods with required signature
static long factr(int n) {
if (n == 0) return 1;
return n * factr(n - 1); }
static long sumr(int n) {
if (n == 0) return 0;
return n + sumr(n - 1);
static long mulr(int n) {
if (n == 0 || n == 1) return n;
return n * mulr(n - 1);

23. Example 2 Defining delegates How to declare delegate reference type?
Globally located within the namespace
delegate long ReturnsLongVal(int par);

24. Example 2 How to create instance of delegate reference type?
Locally located within a method
ReturnsLongVal dlgte;
dlgte = new ReturnsLongVal(factr);
// ReturnsLongVal dlgte = new ReturnsLongVal(factr);

25. Example 2 How to call a method using delegate?
Console.WriteLine(dlgte(5));
Console.WriteLine(dlgte(6));

26. Example 2 How to call a method using delegate?
Console.WriteLine(dlgte(5));
Console.WriteLine(dlgte(6));
dlgte = new ReturnsLongVal(sumr);
Console.WriteLine(dlgte(15));
Console.WriteLine(dlgte(16));

27. Example 2 How to call a method using delegate?
Console.WriteLine(dlgte(5));
Console.WriteLine(dlgte(6));
dlgte = new ReturnsLongVal(sumr);
Console.WriteLine(dlgte(15));
Console.WriteLine(dlgte(16));
dlgte = new ReturnsLongVal(mulr);
Console.WriteLine(dlgte(5+2));
Console.WriteLine(dlgte(6+2));

28. Example 2
Type, compile and run the example 2 application

29. Practice
Task: Write a program to define delegate ref type and declare instance of delegate as Reference to methods with signature: string return data type and two parameters string data type
For indirect access to methods like
static string AddStr(string s1, string s2) {
return s1 + s2; }

30. Practice
Task: Write a program to define delegate ref type and declare instance of delegate as Reference to methods with signature: integer return data type and two parameters integer data type
For indirect access to methods like
static int GCDi(int m, int n)
static int GCDr(int m, int n)

31. Practice
Task: Write a program to define delegate ref type and declare instance of delegate as Reference to methods with signature: double return data type and two parameters: one double data type and one integer data type
For indirect access to methods like
static double Powi(double x, int n)
static double Powr(double x, int n)

32. Relationship of Delegates to Events
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 are 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

33. Event Handling in C#
Form Designer in VStudio does the work for you . Two things added to the program when Double- clicked on a Button control object during design
1) Button Click event is registered as being of interest
2) An event-handler method heading is generated
This two steps process called event wiring process
You associate a method in your program to an event .
Then, this method is automatically called when the event occurs
C# Programming: From Problem Analysis to Program Design

34. 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

35. 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
C# Programming: From Problem Analysis to Program Design

36. ListBox Control Includes number of properties and events
Items property
Used to set initial values
Click on (Collection …) to add items
Name property
Useful to set if you are going to write program statements to manipulate the control
Sorted property
Set to true to avoid having to type values in sorted order
C# Programming: From Problem Analysis to Program Design

37. ListBox Control Properties
Table ListBox properties
C# Programming: From Problem Analysis to Program Design

38. Fill a List Box at Design Time via its String Collection Editor
tasks button
click here to invoke string collection editor

39. 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

40. ListBox Control Methods
Events are methods….Default event for ListBox
Table ListBox methods
C# Programming: From Problem Analysis to Program Design

41. Registering A ListBox Event
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

42. 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

43. 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

44. Multiple selections with ListBox
SelectionMode property
None
One (by default)
private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{ textBox1.Text = listBox1.Text; }
MultiSimple – space bar and clicking the mouse
MultiExtended – Ctrl key, Shift key, arrow keys
C# Programming: From Problem Analysis to Program Design

45. Adding items to ListBox at run time
private void button1_Click(object sender, EventArgs e) {
listBox1.Items.Add(textBox1.Text); }
C# Programming: From Problem Analysis to Program Design

46. Thank You
For
Your Attention!