Event declaration syntax in C Sharp (C#) - Mini Project: Build & Apply

Event Declaration Syntax in C#

📖 Scenario: You are building a simple notification system where a class can notify others when an event happens.

🎯 Goal: Create a class with an event declaration and trigger the event to notify subscribers.

📋 What You'll Learn

Declare an event using the event keyword

Use a delegate type EventHandler for the event

Create a method to raise the event

Subscribe to the event and handle it by printing a message

💡 Why This Matters

🌍 Real World

Events are used in user interfaces, game programming, and many applications to notify parts of a program when something happens.

💼 Career

Understanding event declaration and handling is essential for C# developers working on desktop apps, web apps, and game development.

1

Create a class with an event declaration

Create a class called Notifier and declare an event called OnNotify using the delegate type EventHandler.

C Sharp (C#)

public class Notifier
{
    // Declare the event here
    // Your code here
}

Hint

Use public event EventHandler OnNotify; inside the class.

2

Add a method to raise the event

Inside the Notifier class, add a public method called Notify that raises the OnNotify event if it has subscribers.

C Sharp (C#)

using System;

public class Notifier
{
    public event EventHandler OnNotify;

    // Add Notify method here
    // Your code here
}

Hint

Use the null-conditional operator ?. to safely invoke the event.

3

Subscribe to the event and handle it

Create a Main method. Inside it, create an instance of Notifier called notifier. Subscribe to the OnNotify event with a lambda that prints "Event triggered!".

C Sharp (C#)

using System;

public class Notifier
{
    public event EventHandler OnNotify;

    public void Notify()
    {
        OnNotify?.Invoke(this, EventArgs.Empty);
    }
}

public class Program
{
    public static void Main()
    {
        // Create notifier instance
        // Subscribe to OnNotify event
        // Your code here
    }
}

Hint

Use notifier.OnNotify += (sender, e) => Console.WriteLine("Event triggered!"); to subscribe.

4

Trigger the event and display the output

In the Main method, after subscribing, call the Notify method on notifier to trigger the event and print the message.

C Sharp (C#)

using System;

public class Notifier
{
    public event EventHandler OnNotify;

    public void Notify()
    {
        OnNotify?.Invoke(this, EventArgs.Empty);
    }
}

public class Program
{
    public static void Main()
    {
        Notifier notifier = new Notifier();
        notifier.OnNotify += (sender, e) => Console.WriteLine("Event triggered!");
        // Call Notify method here
        // Your code here
    }
}

Hint

Call notifier.Notify(); to raise the event and see the message.

Practice

1. What keyword is used in C# to declare an event?

easy

A. trigger

B. delegate

C. event

D. notify

Solution

1. Step 1: Understand event declaration basics

   In C#, events are declared using the event keyword followed by a delegate type.

2. Step 2: Identify the correct keyword

   The keyword event specifically marks a member as an event, enabling subscription and notification.

3. Final Answer:

   event -> Option C

4. Quick Check:

   Event declaration uses 'event' keyword [OK]

Hint: Events always start with the 'event' keyword in C# [OK]

Common Mistakes:

• Using 'delegate' instead of 'event' keyword
• Confusing event with method declaration
• Trying to declare event without a delegate type

2. Which of the following is the correct syntax to declare an event named OnClick using a delegate Action?

easy

A. public event Action OnClick;

B. public delegate event Action OnClick;

C. event public Action OnClick();

D. public Action event OnClick;

Solution

1. Step 1: Recall correct event declaration syntax

   The correct syntax is: public event DelegateType EventName; without extra keywords or parentheses.

2. Step 2: Analyze each option

   public event Action OnClick; matches the correct syntax exactly. Options A, B, and D misuse keyword order or add invalid syntax.

3. Final Answer:

   public event Action OnClick; -> Option A

4. Quick Check:

   Correct event syntax = public event Action OnClick; [OK]

Hint: Remember: 'event' comes before delegate type, no parentheses [OK]

Common Mistakes:

• Adding parentheses after event name
• Placing 'event' after delegate type
• Combining 'delegate' and 'event' keywords incorrectly

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

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

medium

A. No output

B. Compilation error

C. Runtime error

D. Started

Solution

1. Step 1: Understand event subscription and invocation

   The event OnStart is subscribed with a lambda that prints "Started". The ?.Invoke() safely calls the event if not null.

2. Step 2: Predict program output

   Since the event has one subscriber, invoking it prints "Started" to the console.

3. Final Answer:

   Started -> Option D

4. Quick Check:

   Subscribed event invoked prints 'Started' [OK]

Hint: ?.Invoke calls event only if not null, so output prints [OK]

Common Mistakes:

• Assuming event is null and no output
• Confusing event invocation syntax
• Expecting compilation or runtime errors

4. Identify the error in the following event declaration and fix it:

public class Button {
    public event Action OnClick();
}

medium

A. Change 'event' to 'delegate'

B. Remove parentheses after OnClick

C. Add 'void' before event keyword

D. Add parentheses after event keyword

Solution

1. Step 1: Analyze event declaration syntax

   Events are declared without parentheses after the event name because they are not methods but event fields.

2. Step 2: Correct the syntax

   Removing parentheses after OnClick fixes the syntax: public event Action OnClick;

3. Final Answer:

   Remove parentheses after OnClick -> Option B

4. Quick Check:

   Events declared without parentheses [OK]

Hint: Events are fields, not methods--no parentheses after name [OK]

Common Mistakes:

• Adding parentheses after event name
• Using 'delegate' keyword instead of 'event'
• Trying to declare event as a method

5. You want to declare an event DataReceived that uses a custom delegate DataHandler accepting a string parameter. Which is the correct declaration?

hard

A. public delegate void DataHandler(string data); public event DataHandler DataReceived;

B. public event void DataHandler(string data); public DataReceived event;

C. public event DataHandler(string data) DataReceived;

D. public DataHandler event DataReceived(string data);

Solution

1. Step 1: Declare the delegate type first

   Custom delegates must be declared separately with the correct signature: public delegate void DataHandler(string data);

2. Step 2: Declare the event using the delegate type

   The event is declared as: public event DataHandler DataReceived; without parentheses.

3. Final Answer:

   public delegate void DataHandler(string data); public event DataHandler DataReceived; -> Option A

4. Quick Check:

   Delegate declared first, event uses delegate type [OK]

Hint: Declare delegate first, then event with delegate type and no parentheses [OK]

Common Mistakes:

• Trying to declare event with parameters directly
• Mixing delegate and event keywords
• Adding parentheses after event name