csharp-basics/11. Delegates and Events.md

# Delegates and Events

![](imgs/11%20Delegates%20and%20Events_0.png)

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# Overview

Delegates

Multicast Delegates

Anonymous Methods

Events

# Delegates

* __Delegates __ are reference type variables that hold a  __reference to a method__  or multiple methods
  * Class objects hold a reference to a class instance, delegate objects hold a reference to a method / methods
* Similar to function pointers in C and C++, or how any function in JavaScript works
* Allows for methods to be passed as variables, useful for creating, for example, events

# Creating a Delegate

Declare a delegate using the following syntax:

delegate \<return type> \<delegate name>(\<parameters>);

Example:

delegate void PrintDelegate(string output);

This creates a new delegate of type void, named PrintDelegate and one parameter of type string

The referenced method return and parameter types have to match the delegate!

# Referencing a Delegate

After creating the delegate, it can be instantiated and the method assigned to it with the method name:

delegate void PrintDelegate(string output);

static void Main(string[] args)

{

void PrintInLower(string text)

{

Console.WriteLine(text.ToLower());

}

PrintDelegate print = PrintInLower;

print("AaaBbbCcc");     // Outputs "aaabbbccc"

}

# Using Multicast Delegates

Delegates can be  __composed of__  multiple methods using the "+" operator

Using the same PrintDelegate delegate as before, we could do this:

delegate void PrintDelegate(string output);

static void Main(string[] args)

{

void PrintInLower(string text)

{

Console.WriteLine(text.ToLower());

}

void PrintInUpper(string text)

{

Console.WriteLine(text.ToUpper());

}

PrintDelegate print = PrintInLower;

print += PrintInUpper;

print("AaaBbbCcc");

}

![](imgs/11%20Delegates%20and%20Events_1.png)

# Using Multicast Delegates (continued)

Methods can be removed from the delegate with the "-" operator:

delegate void PrintDelegate(string output);

static void Main(string[] args)

{

...

print -= PrintInLower;

print("AaaBbbCcc");     // Outputs "AAABBBCCC"

...

}

# Delegates - Example

Let's extend our previous example (without the "-=" part) by creating a new class called DelegateTest, and giving it a constructor that takes a PrintDelegate object as a parameter:

public class DelegateTest

{

public DelegateTest(PrintDelegate printDelegate)

{

printDelegate("This Method Was Called From Another Class!");

}

}

# Delegates - Example (continued)

Now we can create a new DelegateTest object and pass the delegate to the object constructor:

void PrintInLower(string text)

{

Console.WriteLine(text.ToLower());

}

void PrintInUpper(string text)

{

Console.WriteLine(text.ToUpper());

}

// Initialize new delegate which is composed of PrintInLower method

PrintDelegate print = PrintInLower;

// Add PrintInUpper method to the delegate

print += PrintInUpper;

// Send the delegate to the class constructor

DelegateTest delegateTest = new DelegateTest(print);

![](imgs/11%20Delegates%20and%20Events_2.png)

# Anonymous Methods

Delegates can be initialized anonymously (without a specified name)

Anonymous method in variable declaration:

delegate void PrintDelegate(string output);

bool printUpper = true;

PrintDelegate printCheckUpper =

delegate (string text)

{

if (printUpper)

Console.WriteLine(text.ToUpper());

else

Console.WriteLine(text);

};

printCheckUpper("I'm not angry!");        // Outputs I'M NOT ANGRY!

Notice that the actual method that prints the text is not declared anywhere

You can use an empty anonymous method to initialize a delegate which does nothing:

delegate void SomeDelegate();

class Program

{

static void Main(string[] args)

{

// Initialize an empty delegate, add method later...

SomeDelegate myDelegate = new SomeDelegate(delegate { });

}

}

# Events

* Consider the following Game class:
* class Game
* {
* Sound gameOverSound;
* Window gameOverScreen;
* // Some game logic here...
* // ...
* void OnGameOver()
* {
* gameOverSound.Play();   // This plays some sound
* gameOverScreen.Show();  // This shows some screen
* }
* }
* This raises a couple of problems:
  * The Game class has to know about the Sound and Window classes
  * Changes in either of the classes could break the code
  * = Game is dependent of Sound and Window classes
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Tight coupling

# Events (continued)

Events are signals that are raised by a  __Publisher __ and received by a  __Subscriber__

The publisher does not know or care what, if any, object receives the signal

Changes in the subscriber classes do not affect the publisher

# Events - Example

Events are created in two steps:

Declare a delegate

Declare a variable of the delegate with the event keyword

public delegate void GameOver();

class Game

{

public event GameOver GameOverEventHandler;

// Some game logic here...

// ...

}

# Exercise 1: A Rudimentary Event System

Create a console application for controlling a plant treatment system, that has three methods which print the following outputs:

| void ReleaseWater() | Releasing water... |
| :-: | :-: |
| void ReleaseFertilizer() | Releasing fertilizer... |
| void IncreaseTemperature() | Increasing temperature... |

Create a main loop where the user can switch each method on (= add to a delegate) by writing its name. All methods are off by default. (Hint: you can just use switch-case for defining which method should be added to the delegate)

If the user types run, all the methods that are on (= added to the delegate), will be executed.

![](imgs/11%20Delegates%20and%20Events_3.png)