Events

Let’s go through events in C# — a key concept for communication between objects.

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📌 1. What is an Event?

• An event is a way for a class (the publisher) to notify other classes (subscribers) that something happened.

• Events are based on Delegates, which define the signature of the method that handles the event.

• Think of it as a “signal” that something occurred, and any interested object can react to it.

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📌 2. Basic Anatomy of an Event

1. Delegate → defines the method signature for event handlers

2. Event → defines the notification mechanism

3. Subscriber → a method that runs when the event is raised

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Example: Simple Event

using System;
 
public class Alarm
{
    // Step 1: Define delegate
    public delegate void AlarmEventHandler(string message);
 
    // Step 2: Define event
    public event AlarmEventHandler AlarmRaised;
 
    public void TriggerAlarm()
    {
        // Step 3: Raise the event
        AlarmRaised?.Invoke("Alarm triggered!");
    }
}
 
public class Program
{
    static void Main()
    {
        Alarm alarm = new Alarm();
 
        // Step 4: Subscribe to the event
        alarm.AlarmRaised += HandleAlarm;
 
        alarm.TriggerAlarm();
    }
 
    static void HandleAlarm(string msg)
    {
        Console.WriteLine($"Event received: {msg}");
    }
}

Output:

Event received: Alarm triggered!

• AlarmRaised?.Invoke(...) → raises the event (safely checking for subscribers)

• alarm.AlarmRaised += HandleAlarm → subscribes a method to the event

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📌 3. Key Points

Concept	Explanation
Delegate	Defines the signature of the event handler
Event	Publisher sends notifications
Subscriber	Object/method that reacts to the event
Raising event	Uses Invoke() (or shorthand ?.Invoke) to notify subscribers
Syntax sugar	Can use EventHandler delegate for standard events: public event EventHandler MyEvent;

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📌 4. EventHandler Shortcut

• Standardized delegate in .NET:

public event EventHandler SomethingHappened;
 
protected virtual void OnSomethingHappened()
{
    SomethingHappened?.Invoke(this, EventArgs.Empty);
}

• sender → the object that raised the event

• EventArgs → additional event information (can use a derived class for custom data)

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📌 5. Analogy

• Publisher → alarm clock

• Event → alarm bell ringing

• Subscriber → person who hears the bell and reacts

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✅ Tip:

• Events in C# decouple classes — the publisher doesn’t need to know about subscribers.

• Common in UI programming, async notifications, or reactive programming.

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