Event-driven design pattern in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Event-driven design pattern

Start Program

↓

Define Event & Handler

↓

Subscribe Handler to Event

↓

Trigger Event

↓

Event Handler Executes

↓

Continue Program or Exit

The program defines an event and a handler, subscribes the handler to the event, then triggers the event which causes the handler to run.

Execution Sample

C Sharp (C#)

using System;

class Program {
  public static event Action OnJump;
  static void Main() {
    OnJump += () => Console.WriteLine("Jumped!");
    OnJump?.Invoke();
  }
}

This code defines an event OnJump, subscribes a handler that prints 'Jumped!', then triggers the event.

Execution Table

Step	Action	Event State	Handler Called	Output
1	Program starts	OnJump = null	No
2	Subscribe handler to OnJump	OnJump = handler added	No
3	Invoke OnJump event	OnJump has 1 handler	Yes	Jumped!
4	Program ends	OnJump unchanged	No

💡 Program ends after event handler runs once

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
OnJump	null	handler subscribed	handler subscribed	handler subscribed

Key Moments - 3 Insights

Why do we use OnJump += handler instead of OnJump = handler?

Using += adds the handler to the event's invocation list without removing others. This is shown in step 2 of the execution_table where the handler is added, allowing multiple handlers if needed.

What does the ?.Invoke() syntax do?

The ?.Invoke() safely calls the event only if it has handlers subscribed. In step 3, it ensures the handler runs only if OnJump is not null, preventing errors.

When does the event handler run?

The handler runs exactly when the event is invoked, as shown in step 3 where the output 'Jumped!' appears after OnJump?.Invoke() is called.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of OnJump after step 2?

A OnJump has no handlers

B OnJump is null

C OnJump has one handler subscribed

D OnJump is removed

💡 Hint

Check the 'Event State' column in row for step 2 in execution_table

At which step does the event handler print 'Jumped!'?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Output' column in execution_table for when 'Jumped!' appears

If we remove the handler subscription line, what happens when OnJump?.Invoke() is called?

A Program crashes with error

B Nothing happens, no output

C Handler runs anyway

D Event unsubscribes itself

💡 Hint

Refer to the meaning of ?.Invoke() in key_moments and execution_table step 3

Concept Snapshot

Event-driven design uses events to signal actions.
Define an event and handlers.
Subscribe handlers with +=.
Trigger event with ?.Invoke().
Handlers run when event triggers.

Full Transcript

This example shows how event-driven design works in C#. First, the program defines an event called OnJump. Then, it subscribes a handler that prints 'Jumped!' when the event happens. When the program calls OnJump?.Invoke(), it triggers the event, causing the handler to run and print the message. The ?.Invoke() syntax ensures the event is only called if there are handlers subscribed, preventing errors. This pattern lets programs respond to actions flexibly by running code only when events occur.

Practice

1. What is the main role of an event subscriber in the event-driven design pattern?

easy

A. To manage the program's user interface layout

B. To listen for events and respond when they occur

C. To store data for the program's use

D. To create and send events to other parts of the program

Solution

1. Step 1: Understand the roles in event-driven design

   In event-driven design, publishers send events and subscribers listen for these events.

2. Step 2: Identify subscriber's responsibility

   The subscriber's job is to react or respond when an event happens, meaning it listens and acts accordingly.

3. Final Answer:

   To listen for events and respond when they occur -> Option B

4. Quick Check:

   Subscriber listens and reacts = D [OK]

Hint: Subscribers always listen and react to events [OK]

Common Mistakes:

• Confusing subscriber with publisher
• Thinking subscriber creates events
• Assuming subscriber manages UI layout

2. Which of the following is the correct syntax to declare an event in C#?

easy

A. public void event MyEvent();

B. event public MyEvent EventHandler;

C. public event EventHandler MyEvent;

D. EventHandler public event MyEvent;

Solution

1. Step 1: Recall event declaration syntax in C#

   Events are declared with the keyword 'event' followed by a delegate type, then the event name.

2. Step 2: Check each option

   public event EventHandler MyEvent; matches the correct syntax: 'public event EventHandler MyEvent;'. Others have wrong order or keywords.

3. Final Answer:

   public event EventHandler MyEvent; -> Option C

4. Quick Check:

   Correct event declaration = B [OK]

Hint: Event declaration: 'public event DelegateType EventName;' [OK]

Common Mistakes:

• Mixing method and event syntax
• Wrong keyword order
• Missing 'event' keyword

3. What will be the output of the following C# code?

using System;
class Program {
  public static event Action OnNotify;
  static void Main() {
    OnNotify += () => Console.WriteLine("Event triggered");
    OnNotify?.Invoke();
  }
}

medium

A. Event triggered

B. No output

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand event subscription and invocation

   The event OnNotify is subscribed with a lambda that prints "Event triggered". The event is then invoked safely with ?.Invoke().

2. Step 2: Predict output

   Since OnNotify has a subscriber, invoking it calls the lambda, printing "Event triggered" to the console.

3. Final Answer:

   Event triggered -> Option A

4. Quick Check:

   Subscribed event invoked prints message = A [OK]

Hint: Subscribed event invocation prints output [OK]

Common Mistakes:

• Ignoring null-conditional operator ?.Invoke()
• Expecting no output without subscription
• Confusing event with method call syntax

4. Identify the error in this C# event-driven code snippet:

public class Publisher {
  public event EventHandler OnChange;
  public void Raise() {
    OnChange();
  }
}

medium

A. OnChange event is invoked without checking for null

B. EventHandler delegate is not declared

C. Raise method should be static

D. Event should be private

Solution

1. Step 1: Review event invocation rules

   Events can be null if no subscribers exist. Invoking without null check causes runtime error.

2. Step 2: Identify the error in code

   OnChange() is called directly without checking if OnChange is null, risking NullReferenceException.

3. Final Answer:

   OnChange event is invoked without checking for null -> Option A

4. Quick Check:

   Invoke event safely with null check = C [OK]

Hint: Always check event for null before invoking [OK]

Common Mistakes:

• Calling event without null check
• Assuming event is never null
• Misunderstanding event access modifiers

5. You want to design a system where multiple subscribers react differently to a button click event. Which approach best uses the event-driven pattern in C# to achieve this?

hard

A. Make subscribers poll the button state continuously in a loop

B. Create a list of methods and call them manually without events

C. Use a static method to handle all button clicks centrally

D. Declare a public event, let subscribers add their handlers, and invoke event on button click

Solution

1. Step 1: Understand event-driven design for multiple subscribers

   Events allow multiple subscribers to register handlers that get called automatically when the event fires.

2. Step 2: Evaluate options for best practice

   Declare a public event, let subscribers add their handlers, and invoke event on button click uses a public event and lets subscribers add handlers, which is the standard event-driven approach. Others are less efficient or not event-driven.

3. Final Answer:

   Declare a public event, let subscribers add their handlers, and invoke event on button click -> Option D

4. Quick Check:

   Use events for multiple subscriber reactions = A [OK]

Hint: Use events to notify multiple subscribers automatically [OK]

Common Mistakes:

• Manually calling subscriber methods instead of using events
• Using polling instead of event notifications
• Centralizing all logic in static methods