Event declaration syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this event declaration and usage?

Consider the following C# code snippet. What will be printed when the program runs?

C Sharp (C#)

using System;

class Program {
    public delegate void Notify();
    public event Notify OnNotify;

    public void Trigger() {
        OnNotify?.Invoke();
    }

    static void Main() {
        Program p = new Program();
        p.OnNotify += () => Console.WriteLine("Event triggered!");
        p.Trigger();
    }
}

A Runtime error: NullReferenceException

B No output

C Compile-time error: event must be declared inside a class

D Event triggered!

💡 Hint

Check if the event is subscribed before invoking.

The event OnNotify is subscribed with a lambda that prints "Event triggered!". The Trigger method invokes the event safely using the null-conditional operator ?., so the output is printed.

❓ Predict Output

intermediate

What error occurs with this event declaration?

What error will this code produce?

C Sharp (C#)

using System;

class Program {
    public event Action OnAction = () => {};

    static void Main() {
        Program p = new Program();
        p.OnAction();
    }
}

A Runtime error: NullReferenceException

B Compile-time error: Events cannot have initializers

C No output, runs fine

D Compile-time error: Missing delegate declaration

💡 Hint

Events cannot be assigned directly with initial values.

In C#, events cannot be initialized directly with a delegate. This causes a compile-time error.

🔧 Debug

advanced

Why does this event invocation cause a runtime error?

Examine the code below. Why does it throw an exception at runtime?

C Sharp (C#)

using System;

class Program {
    public event Action OnEvent;

    public void Raise() {
        OnEvent();
    }

    static void Main() {
        Program p = new Program();
        p.Raise();
    }
}

A NullReferenceException because OnEvent has no subscribers

B Compile-time error: event cannot be invoked outside the declaring class

C No output, runs fine

D InvalidOperationException because event is null

💡 Hint

Check if the event is null before invoking.

The event OnEvent has no subscribers, so it is null. Invoking it directly causes a NullReferenceException.

📝 Syntax

advanced

Which option correctly declares an event with a custom delegate?

Which of the following is the correct way to declare an event using a custom delegate in C#?

A

delegate public void MyEventHandler(string message);
event MyEventHandler EventOccurred;

B

public event delegate void MyEventHandler(string message);
public EventOccurred MyEventHandler;

C

public delegate void MyEventHandler(string message);
public event MyEventHandler EventOccurred;

D

public void delegate MyEventHandler(string message);
public event EventOccurred MyEventHandler;

💡 Hint

Remember the order: delegate declaration first, then event declaration.

Option C correctly declares a delegate type and then an event of that delegate type.

🚀 Application

expert

How many subscribers does the event have after this code runs?

Given the following code, how many methods are subscribed to the OnChange event after execution?

C Sharp (C#)

using System;

class Program {
    public event Action OnChange;

    public void Subscribe() {
        OnChange += Handler1;
        OnChange += Handler2;
        OnChange -= Handler1;
        OnChange += Handler3;
    }

    void Handler1() { }
    void Handler2() { }
    void Handler3() { }

    static void Main() {
        Program p = new Program();
        p.Subscribe();
        Console.WriteLine(p.OnChange?.GetInvocationList().Length ?? 0);
    }
}

A 2

B 3

C 1

D 0

💡 Hint

Consider the += and -= operations on the event.

Handler1 is added then removed, Handler2 and Handler3 remain subscribed, so 2 subscribers remain.

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