EventHandler delegate pattern in C Sharp (C#) - Step-by-Step Execution

Concept Flow - EventHandler delegate pattern

Define EventHandler delegate

↓

Create event using EventHandler

↓

Subscribe method to event

↓

Trigger event (Raise event)

↓

Invoke subscribed methods

↓

Methods respond to event

This flow shows how an EventHandler delegate is defined, an event is created and subscribed to, then triggered to call the subscribed methods.

Execution Sample

C Sharp (C#)

using System;

class Program {
  static event EventHandler MyEvent;

  static void Main() {
    MyEvent += Handler;
    MyEvent?.Invoke(null, EventArgs.Empty);
  }

  static void Handler(object? sender, EventArgs e) {
    Console.WriteLine("Event triggered!");
  }
}

This code defines an event using EventHandler, subscribes a method, then triggers the event to print a message.

Execution Table

Step	Action	EventHandler MyEvent	Output
1	Start program	null
2	Subscribe Handler to MyEvent	Handler method added
3	Invoke MyEvent	Handler method called	Event triggered!
4	Program ends	Handler method remains subscribed

💡 Program ends after event is triggered and Handler method runs

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
MyEvent	null	Handler method subscribed	Handler method subscribed	Handler method subscribed

Key Moments - 3 Insights

Why is MyEvent null at the start and not empty?

At start, no methods are subscribed, so MyEvent is null, not an empty list. See execution_table step 1.

What does the ?.Invoke syntax do?

It safely calls the event only if it's not null, preventing errors if no subscribers exist. See execution_table step 3.

Why does the Handler method receive two parameters?

EventHandler delegate requires sender and EventArgs parameters, even if unused. See code in execution_sample.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of MyEvent after step 2?

A Handler method subscribed

B null

C Empty delegate

D Event triggered!

💡 Hint

Check the 'EventHandler MyEvent' column in row for step 2.

At which step does the program output "Event triggered!"?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Output' column in the execution_table.

If no method was subscribed to MyEvent, what would happen at step 3?

A Program crashes with null reference error

B Handler method is called anyway

C Nothing happens, no output

D EventHandler becomes empty delegate

💡 Hint

The ?.Invoke syntax prevents calling null delegate, so no output occurs if no subscribers.

Concept Snapshot

EventHandler delegate pattern in C#:
- Define event: event EventHandler MyEvent;
- Subscribe: MyEvent += Handler;
- Trigger: MyEvent?.Invoke(sender, EventArgs.Empty);
- Handler signature: void Handler(object? sender, EventArgs e)
- ?.Invoke avoids null errors
- Used for simple event notification

Full Transcript

This example shows how to use the EventHandler delegate pattern in C#. First, an event named MyEvent is declared using the EventHandler delegate type. Initially, MyEvent is null because no methods are subscribed. Then, the Handler method is subscribed to MyEvent using +=. When the event is triggered by calling MyEvent?.Invoke(null, EventArgs.Empty), the Handler method runs and prints "Event triggered!". The ?.Invoke syntax ensures the event is only called if it has subscribers, preventing errors. The Handler method must accept two parameters: sender and EventArgs, as required by EventHandler. This pattern allows simple event notification and handling in C# programs.