The document discusses delegates, events, and extension methods in C#. It explains that delegates allow functions to be passed as parameters and can point to methods. Events use delegates to call subscriber methods when an event is raised. Extension methods extend existing classes with new methods without modifying the original class. The document provides examples of how to use delegates to handle events, attach multiple event handlers, and create anonymous methods. It also demonstrates how to write an extension method to add new functionality to the string class.

1. Mohammad Shaker
mohammadshaker.com
C# Programming Course
@ZGTRShaker
2011, 2012, 2013, 2014
C# Starter
L06 – Delegates, Event Handling and
Extension Methods

2. Delegates

3. Delegates
Have you ever thought to pass a function as a parameter?

4. Delegates
• Delegate
– A delegate is an object that points to a function
– is basically a function pointer, C++!
– "Why do I need a reference to a method?"
• The answer boils down to giving you maximum flexibility to implement any functionality you want at
runtime.
– You can use delegate to pass a function as a parameter

5. Delegate Example
public class MagicNumber
{
public int Number { get; set; }
}
public void Increment(MagicNumber magicNumber)
{
magicNumber.Number++;
}
public void Decrement(MagicNumber magicNumber)
{
magicNumber.Number--;
}
public delegate void MagicNumberModifier (MagicNumber magicNumber);
Delegate Declaration

6. Delegate Example
• Now let’s make a list of MagicNumbers
• And a member like this:
List<MagicNumber> magicNumbers = new List<MagicNumber>();
magicNumbers.Add(new MagicNumber() { Number = 4});
magicNumbers.Add(new MagicNumber() { Number = 6});
magicNumbers.Add(new MagicNumber() { Number = 9});
magicNumbers.Add(new MagicNumber() { Number = 13});
magicNumbers.Add(new MagicNumber() { Number = 20});
public static void ApplyModifierToAll(MagicNumberModifier modifier, List<MagicNumber>
magicNumbers)
{
foreach(MagicNumber number in magicNumbers)
{
modifier(number);
}
}
Delegate!
public delegate void MagicNumberModifier (MagicNumber magicNumber);

7. Delegate Example
• Lastly, just call the delegate and let the magic happens
public static void Main(string[] args)
{
List<MagicNumber> magicNumbers = new List<MagicNumber>();
magicNumbers.Add(new MagicNumber() { Number = 4, IsMagic = true });
magicNumbers.Add(new MagicNumber() { Number = 6, IsMagic = false });
magicNumbers.Add(new MagicNumber() { Number = 9, IsMagic = true });
magicNumbers.Add(new MagicNumber() { Number = 13, IsMagic = false });
magicNumbers.Add(new MagicNumber() { Number = 20, IsMagic = true });
ApplyModifierToAll(Increment, magicNumbers);
ApplyModifierToAll(Magicify, magicNumbers);
}

8. MultiCasting-Delegates
• Note!
– If you chain delegates that return values, you may run into some problems. The way C#
handles that situation is by returning the value of only the last function inside the delegate. All
other return values are discarded. So don’t return things that are absolutely vital to your
program if you know the function is going inside a chained delegate.

9. Event Handling

10. Event Handling
Delegates and Event Handling

11. Event Handling
Raise Events when Needed

12. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}

13. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}

14. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}

15. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}
The event handler
Raising the event

16. Attaching and Detaching Events

17. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}
class Program
{
public static void Main()
{
new Program().Test();
}
private void Test()
{
Point point = new Point();
// Here's the real important line:
point.PointChanged += HandlePointChanged;
// Now if we change the point, the PointChanged event will get raised,
// and HandlePointChanged will get called.
point.X = 3;
}
public void HandlePointChanged(object sender, EventArgs eventArgs)
{
// Do something intelligent when the point changes. Perhaps redraw the GUI,
// or update another data structure, or anything else you can think of.
}
}

18. Event Handling
How it works now..

19. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}
class Program
{
public static void Main()
{
new Program().Test();
}
private void Test()
{
Point point = new Point();
// Here's the real important line:
point.PointChanged += HandlePointChanged;
// Now if we change the point, the PointChanged event will get raised,
// and HandlePointChanged will get called.
point.X = 3;
}
public void HandlePointChanged(object sender, EventArgs eventArgs)
{
// Do something intelligent when the point changes. Perhaps redraw the GUI,
// or update another data structure, or anything else you can think of.
}
}
When a point is Set the
method OnPointChanged
will be called

20. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}
class Program
{
public static void Main()
{
new Program().Test();
}
private void Test()
{
Point point = new Point();
// Here's the real important line:
point.PointChanged += HandlePointChanged;
// Now if we change the point, the PointChanged event will get raised,
// and HandlePointChanged will get called.
point.X = 3;
}
public void HandlePointChanged(object sender, EventArgs eventArgs)
{
// Do something intelligent when the point changes. Perhaps redraw the GUI,
// or update another data structure, or anything else you can think of.
}
}
Which will consequently
raise the event by calling it
(checking if the delegate is
null for the first time)

21. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}
class Program
{
public static void Main()
{
new Program().Test();
}
private void Test()
{
Point point = new Point();
// Here's the real important line:
point.PointChanged += HandlePointChanged;
// Now if we change the point, the PointChanged event will get raised,
// and HandlePointChanged will get called.
point.X = 3;
}
public void HandlePointChanged(object sender, EventArgs eventArgs)
{
// Do something intelligent when the point changes. Perhaps redraw the GUI,
// or update another data structure, or anything else you can think of.
}
}
And consequently, calling
the delegate that points to
the programmer function

22. public class Point
{
private double x;
private double y;
public double X
{
get
{
return x;
}
set
{
x = value;
OnPointChanged();
}
}
public double Y
{
get
{
return y;
}
set
{
y = value;
OnPointChanged();
}
}
public event EventHandler PointChanged;
public void OnPointChanged()
{
if(PointChanged != null)
{
PointChanged(this, EventArgs.Empty);
}
}
}
class Program
{
public static void Main()
{
new Program().Test();
}
private void Test()
{
Point point = new Point();
// Here's the real important line:
point.PointChanged += HandlePointChanged;
// Now if we change the point, the PointChanged event will get raised,
// and HandlePointChanged will get called.
point.X = 3;
}
public void HandlePointChanged(object sender, EventArgs eventArgs)
{
// Do something intelligent when the point changes. Perhaps redraw the GUI,
// or update another data structure, or anything else you can think of.
}
}
And consequently, calling
the delegate that points to
the programmer function

23. Attaching/Detaching Multiple Events
• The event can easily be removed/changed/added at runtime by -= or +=
• An event can also add multiple function pointers (delegates at the same time),
pointing at all of them at the same time and calling all of them when the event is
raised!
point.PointChanged -= HandlePointChanged;
point.PointChanged += HandlePointChanged;
point.PointChanged += HandlePotatoPositionChange;

24. Anonymous Methods

25. Anonymous Methods
• An anonymous method is a method without a name
– which is why it is called anonymous.
• You don't declare anonymous methods like regular methods.
– Instead they get hooked up directly to events.

26. Anonymous Methods
using System.Windows.Forms;
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Controls.Add(buttonHello);
}
}

27. Anonymous Methods
using System.Windows.Forms;
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Controls.Add(buttonHello);
}
}

28. Anonymous Methods
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Button btnGoodBye = new Button();
btnGoodBye.Text = "Goodbye";
btnGoodBye.Left = buttonHello.Width + 5;
btnGoodBye.Click +=
delegate(object sender, EventArgs e)
{
string message = (sender as Button).Text;
MessageBox.Show(message);
};
Controls.Add(buttonHello);
Controls.Add(btnGoodBye);
}
}

29. Anonymous Methods
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Button btnGoodBye = new Button();
btnGoodBye.Text = "Goodbye";
btnGoodBye.Left = buttonHello.Width + 5;
btnGoodBye.Click +=
delegate(object sender, EventArgs e)
{
string message = (sender as Button).Text;
MessageBox.Show(message);
};
Controls.Add(buttonHello);
Controls.Add(btnGoodBye);
}
}

30. Anonymous Methods
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Button btnGoodBye = new Button();
btnGoodBye.Text = "Goodbye";
btnGoodBye.Left = buttonHello.Width + 5;
btnGoodBye.Click +=
delegate(object sender, EventArgs e)
{
string message = (sender as Button).Text;
MessageBox.Show(message);
};
Controls.Add(buttonHello);
Controls.Add(btnGoodBye);
}
}

31. Anonymous Methods
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Button btnGoodBye = new Button();
btnGoodBye.Text = "Goodbye";
btnGoodBye.Left = buttonHello.Width + 5;
btnGoodBye.Click +=
delegate(object sender, EventArgs e)
{
string message = (sender as Button).Text;
MessageBox.Show(message);
};
Controls.Add(buttonHello);
Controls.Add(btnGoodBye);
}
}

32. Anonymous Methods
public partial class Form1 : Form
{
public Form1()
{
Button buttonHello = new Button();
buttonHello.Text = "Hello";
buttonHello.Click +=
delegate
{
MessageBox.Show("Hello");
};
Button btnGoodBye = new Button();
btnGoodBye.Text = "Goodbye";
btnGoodBye.Left = buttonHello.Width + 5;
btnGoodBye.Click +=
delegate(object sender, EventArgs e)
{
string message = (sender as Button).Text;
MessageBox.Show(message);
};
Controls.Add(buttonHello);
Controls.Add(btnGoodBye);
}
}

33. Extension Methods

34. Extension Methods
Extension methods are a way to create a method that feels like it is part of a specific class
(like, the string class) when you don't actually have access to that class to modify, and add the
method.

35. Extension Methods
It’s a way to add functionalities to classes you don’t own

36. Extension Methods
It’s a way to add functionalities to classes you don’t own
that’s why they are called Extension Methods

37. Extension Methods
• Extension methods must be added in static classes (where methods in that static
class should also be static)
• Now, Let’s extend the String class with a ToRandomCase method that
randomize the letter cases in a given string.

38. Extension Methods
public static class StringExtensions
{
private static Random random = new Random();
public static string ToRandomCase(this string text)
{
string result = "";
for (int index = 0; index < text.Length; index++)
{
if (random.Next(2) == 0)
{
result += text.Substring(index, 1).ToUpper();
}
else
{
result += text.Substring(index, 1).ToLower();
}
}
return result;
}
}
Static class

39. Extension Methods
public static class StringExtensions
{
private static Random random = new Random();
public static string ToRandomCase(this string text)
{
string result = "";
for (int index = 0; index < text.Length; index++)
{
if (random.Next(2) == 0)
{
result += text.Substring(index, 1).ToUpper();
}
else
{
result += text.Substring(index, 1).ToLower();
}
}
return result;
}
}
Static data member

40. Extension Methods
public static class StringExtensions
{
private static Random random = new Random();
public static string ToRandomCase(this string text)
{
string result = "";
for (int index = 0; index < text.Length; index++)
{
if (random.Next(2) == 0)
{
result += text.Substring(index, 1).ToUpper();
}
else
{
result += text.Substring(index, 1).ToLower();
}
}
return result;
}
}
Static method

41. Extension Methods
public static class StringExtensions
{
private static Random random = new Random();
public static string ToRandomCase(this string text)
{
string result = "";
for (int index = 0; index < text.Length; index++)
{
if (random.Next(2) == 0)
{
result += text.Substring(index, 1).ToUpper();
}
else
{
result += text.Substring(index, 1).ToLower();
}
}
return result;
}
}
The extended class (string)
with the this keyword

42. Extension Methods
public static class StringExtensions
{
private static Random random = new Random();
public static string ToRandomCase(this string text)
{
string result = "";
for (int index = 0; index < text.Length; index++)
{
if (random.Next(2) == 0)
{
result += text.Substring(index, 1).ToUpper();
}
else
{
result += text.Substring(index, 1).ToLower();
}
}
return result;
}
}
A simple randomization
algorithm

43. Extension Methods
• Now just call the method!
public static void Main()
{
string message = "Do you wish me a good morning, or mean that it is a good morning whether I
want it or not; or that you feel good this morning; or that it is a morning to be good on?";
Console.WriteLine(message.ToRandomCase());
}
Do You WisH mE A gOOD MOrniNG, Or mEan that it iS a GOOD moRnIng wHether i Want It or noT; Or that yOu
feeL goOD THiS morning; OR tHaT it iS A MOrNINg to be GoOd On?
Press any key to continue . . .