Delegates as callback pattern in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of delegate callback invocation

What is the output of this C# program that uses a delegate as a callback?

C Sharp (C#)

using System;

public delegate void Notify(string message);

class Program
{
    static void Main()
    {
        Notify notifier = PrintMessage;
        notifier("Hello from delegate!");
    }

    static void PrintMessage(string msg)
    {
        Console.WriteLine(msg);
    }
}

A Hello from delegate!

B System.Delegate

C Compile-time error

D Runtime exception

💡 Hint

Look at how the delegate is assigned and invoked.

The delegate Notify points to the method PrintMessage. When invoked, it calls PrintMessage with the given string, printing it.

❓ Predict Output

intermediate

Delegate callback with multiple methods

What will be the output of this C# program using a multicast delegate as a callback?

C Sharp (C#)

using System;

public delegate void Notify(string message);

class Program
{
    static void Main()
    {
        Notify notifier = PrintMessage;
        notifier += PrintMessageUpper;
        notifier("Hello!");
    }

    static void PrintMessage(string msg)
    {
        Console.WriteLine(msg);
    }

    static void PrintMessageUpper(string msg)
    {
        Console.WriteLine(msg.ToUpper());
    }
}

A HELLO!\nHello!

B Compile-time error

C Hello!

D Hello!\nHELLO!

💡 Hint

Multicast delegates call all methods in their invocation list.

The delegate notifier calls both PrintMessage and PrintMessageUpper in order, so both lines print.

🔧 Debug

advanced

Identify the runtime error in delegate callback

What error does this C# program produce when run?

C Sharp (C#)

using System;

public delegate void Notify(string message);

class Program
{
    static void Main()
    {
        Notify notifier = null;
        notifier("Test message");
    }
}

A NullReferenceException

B Compile-time error: delegate not assigned

C ArgumentNullException

D No error, prints 'Test message'

💡 Hint

What happens if you call a delegate variable that is null?

Calling a null delegate causes a NullReferenceException at runtime because there is no method to invoke.

🧠 Conceptual

advanced

Delegate callback pattern usage

Which statement best describes the use of delegates as callbacks in C#?

A Delegates automatically run all methods in parallel threads.

B Delegates allow methods to be passed as parameters and called later, enabling flexible callback patterns.

C Delegates are used only to store multiple unrelated methods without parameters.

D Delegates replace all event handling mechanisms in C#.

💡 Hint

Think about what delegates enable in terms of method references and callbacks.

Delegates are type-safe references to methods. They can be passed around and invoked later, which is the core of callback patterns.

📝 Syntax

expert

Correct delegate declaration for callback with return value

Which option correctly declares a delegate type for a callback that takes an int and returns a string?

A delegate string Callback(int value) {}

B public delegate void Callback(int value) => string;

C public delegate string Callback(int value);

D public Callback delegate string(int value);

💡 Hint

Remember the syntax for declaring delegates: public delegate ReturnType Name(ParameterList);

Option C uses the correct syntax for declaring a delegate with a return type and parameter.