Raising events safely in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this event invocation code?

Consider the following C# code that raises an event safely. What will be printed when OnDataReceived is called?

C Sharp (C#)

using System;

class Program
{
    public event EventHandler DataReceived;

    public void OnDataReceived()
    {
        EventHandler handler = DataReceived;
        if (handler != null)
        {
            handler(this, EventArgs.Empty);
        }
    }

    static void Main()
    {
        Program p = new Program();
        p.DataReceived += (s, e) => Console.WriteLine("Event triggered");
        p.OnDataReceived();
    }
}

A Event triggered

B No output

C NullReferenceException at runtime

D Compilation error due to event invocation

💡 Hint

Think about how the event is copied to a local variable before invocation.

The event is copied to a local variable 'handler' to avoid race conditions. Since the event has one subscriber, invoking 'handler' calls the subscriber and prints 'Event triggered'.

❓ Predict Output

intermediate

What happens if you invoke an event without null check?

What will happen when the following code runs?

C Sharp (C#)

using System;

class Program
{
    public event EventHandler DataReceived;

    public void OnDataReceived()
    {
        DataReceived(this, EventArgs.Empty);
    }

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

A Throws NullReferenceException at runtime

B Prints 'Event triggered'

C Compilation error due to missing event handler

D No output and no error

💡 Hint

What happens if you call an event with no subscribers?

Since no subscriber is attached, the event delegate is null. Invoking it directly causes a NullReferenceException.

🔧 Debug

advanced

Identify the thread-safety issue in this event raising code

What is the problem with this code when raising events in a multithreaded environment?

C Sharp (C#)

using System;

class Program
{
    public event EventHandler DataReceived;

    public void OnDataReceived()
    {
        if (DataReceived != null)
        {
            DataReceived(this, EventArgs.Empty);
        }
    }
}

A The event is invoked twice accidentally

B The event can become null after the null check but before invocation, causing NullReferenceException

C The event handler list is modified during invocation causing InvalidOperationException

D No thread-safety issues present

💡 Hint

Consider what happens if another thread unsubscribes between the null check and invocation.

Between the null check and invocation, another thread might unsubscribe all handlers, making the event null and causing a NullReferenceException.

❓ Predict Output

advanced

What is the output of this event invocation with multiple subscribers?

Given the code below, what will be printed when OnDataReceived is called?

C Sharp (C#)

using System;

class Program
{
    public event EventHandler DataReceived;

    public void OnDataReceived()
    {
        EventHandler handler = DataReceived;
        handler?.Invoke(this, EventArgs.Empty);
    }

    static void Main()
    {
        Program p = new Program();
        p.DataReceived += (s, e) => Console.WriteLine("First subscriber");
        p.DataReceived += (s, e) => Console.WriteLine("Second subscriber");
        p.OnDataReceived();
    }
}

A Only First subscriber

B Second subscriber\nFirst subscriber

C First subscriber\nSecond subscriber

D No output

💡 Hint

Events call subscribers in the order they were added.

Events invoke all subscribers in the order they were added. Both subscribers print their messages in sequence.

❓ Predict Output

expert

What is the output and why? (Event with custom delegate and null propagation)

Analyze the code below. What will be the output when RaiseEvent is called?

C Sharp (C#)

using System;

class Program
{
    public delegate void CustomEventHandler(string message);
    public event CustomEventHandler? CustomEvent;

    public void RaiseEvent()
    {
        CustomEvent?.Invoke("Hello World");
    }

    static void Main()
    {
        Program p = new Program();
        CustomEventHandler handler = msg => Console.WriteLine($"Received: {msg}");
        p.CustomEvent += handler;
        p.RaiseEvent();

        p.CustomEvent -= handler;
        p.RaiseEvent();
    }
}

A Compilation error due to null assignment to event

B

Received: Hello World
Throws NullReferenceException on second call

C

No output
Throws NullReferenceException on second call

D

Received: Hello World
(no output on second call)

💡 Hint

Events with null propagation operator do not throw if no subscribers.

The null propagation operator safely invokes the event only if it has subscribers. Unsubscribing the handler makes the event null, so the second call does nothing.