Publisher-subscriber execution model in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this simple publisher-subscriber example?

Consider the following C# code implementing a basic publisher-subscriber pattern. What will be printed when the program runs?

C Sharp (C#)

using System;

public class Publisher
{
    public event Action OnChange;

    public void RaiseEvent()
    {
        OnChange?.Invoke();
    }
}

public class Subscriber
{
    public void Respond()
    {
        Console.WriteLine("Subscriber received event.");
    }
}

public class Program
{
    public static void Main()
    {
        Publisher pub = new Publisher();
        Subscriber sub = new Subscriber();
        pub.OnChange += sub.Respond;
        pub.RaiseEvent();
    }
}

A Subscriber received event.

B No output

C Runtime error: NullReferenceException

D Compile error: event invocation not allowed

💡 Hint

Look at how the event is subscribed and invoked.

The subscriber method Respond is attached to the publisher's event OnChange. When RaiseEvent is called, it invokes OnChange, triggering Respond and printing the message.

❓ Predict Output

intermediate

What happens if no subscriber is attached?

What will be the output or behavior of this code when RaiseEvent is called but no subscriber is attached?

C Sharp (C#)

using System;

public class Publisher
{
    public event Action OnChange;

    public void RaiseEvent()
    {
        OnChange?.Invoke();
        Console.WriteLine("Event raised.");
    }
}

public class Program
{
    public static void Main()
    {
        Publisher pub = new Publisher();
        pub.RaiseEvent();
    }
}

A Compile error: event invocation not allowed

B Runtime error: NullReferenceException

C No output

D Event raised.

💡 Hint

Check the null-conditional operator usage.

The event OnChange is null because no subscriber is attached. The null-conditional operator ?. prevents invocation if null, so no error occurs. The line Console.WriteLine("Event raised."); still runs.

🔧 Debug

advanced

Identify the error in this publisher-subscriber code

What error will this code produce when compiled or run?

C Sharp (C#)

using System;

public class Publisher
{
    public event Action OnChange;

    public void RaiseEvent()
    {
        OnChange();
    }
}

public class Program
{
    public static void Main()
    {
        Publisher pub = new Publisher();
        pub.RaiseEvent();
    }
}

A Runtime error: NullReferenceException

B Compile error: event invocation not allowed

C No output

D Runtime error: StackOverflowException

💡 Hint

Consider what happens if the event has no subscribers and is invoked directly.

The event OnChange is null because no subscriber is attached. Invoking it directly without checking for null causes a NullReferenceException at runtime.

🧠 Conceptual

advanced

Which statement about publisher-subscriber events is true?

Choose the correct statement about the publisher-subscriber pattern using C# events.

A An event can be invoked from outside the class that declares it.

B Subscribers can only attach methods with matching delegate signatures.

C Events automatically queue messages if subscribers are busy.

D Multiple events cannot share the same delegate type.

💡 Hint

Think about how delegates and events work in C#.

Subscribers must attach methods that match the delegate signature of the event. Events cannot be invoked from outside their declaring class. Events do not queue messages automatically. Multiple events can share the same delegate type.

❓ Predict Output

expert

What is the output when multiple subscribers are attached?

Given this code with two subscribers attached to the same event, what will be printed?

C Sharp (C#)

using System;

public class Publisher
{
    public event Action OnChange;

    public void RaiseEvent()
    {
        OnChange?.Invoke();
    }
}

public class Program
{
    public static void Main()
    {
        Publisher pub = new Publisher();
        pub.OnChange += () => Console.WriteLine("First subscriber called.");
        pub.OnChange += () => Console.WriteLine("Second subscriber called.");
        pub.RaiseEvent();
    }
}

A No output

B First subscriber called.

C

First subscriber called.
Second subscriber called.

D

Second subscriber called.
First subscriber called.

💡 Hint

Events invoke subscribers in the order they were added.

The event OnChange calls subscribers in the order they were attached. So the first lambda prints first, then the second.