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.