Action delegate type in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Action delegate with multiple methods

What is the output of this C# code using an Action delegate with multiple methods?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        Action action = Method1;
        action += Method2;
        action();
    }

    static void Method1() {
        Console.WriteLine("Hello");
    }

    static void Method2() {
        Console.WriteLine("World");
    }
}

A World\nHello

B Hello\nWorld

C Hello

D Compilation error

💡 Hint

Think about how Action delegates combine multiple methods and invoke them in order.

The Action delegate can hold references to multiple methods. When invoked, it calls each method in the order they were added. So Method1 prints "Hello" first, then Method2 prints "World".

❓ Predict Output

intermediate

Action delegate with parameters output

What will be printed when this C# program runs?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        Action<string> greet = name => Console.WriteLine($"Hi, {name}!");
        greet("Alice");
    }
}

A Hi, Alice!

B Compilation error

C Hi, !

D Hi, {name}!

💡 Hint

Look at how the lambda uses the parameter to print the greeting.

The Action delegate takes a string parameter. The lambda prints "Hi, " followed by the name passed. So it prints "Hi, Alice!".

🔧 Debug

advanced

Why does this Action delegate cause a runtime error?

Consider this code snippet. Why does it throw a NullReferenceException at runtime?

C Sharp (C#)

using System;

class Program {
    static Action action;

    static void Main() {
        action();
    }
}

A The Action delegate 'action' is null because it was never assigned a method.

B The Action delegate cannot be static.

C The Main method must return void, not int.

D The Action delegate requires parameters to be invoked.

💡 Hint

Check if the delegate variable was assigned before calling it.

The 'action' delegate is declared but never assigned a method. Calling it causes a NullReferenceException because it points to null.

📝 Syntax

advanced

Identify the syntax error in Action delegate declaration

Which option contains a syntax error when declaring an Action delegate?

A Action<int> printNumber = n => Console.WriteLine(n);

B Action printHello = () => Console.WriteLine("Hello");

C Action<int> action = (int n) => { Console.WriteLine(n); }

D Action<string> greet = (string name) => Console.WriteLine(name);

💡 Hint

Check the syntax inside the lambda body for missing punctuation.

Option C is missing a semicolon after Console.WriteLine(n) inside the lambda block, causing a syntax error.

🚀 Application

expert

Using Action delegate to modify a list

Given this code, what is the final content of the list 'numbers' after invoking the Action delegate?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        List<int> numbers = new() {1, 2, 3};
        Action<List<int>> addNumbers = list => {
            list.Add(4);
            list.Add(5);
        };
        addNumbers(numbers);
        numbers.Remove(2);
        addNumbers(numbers);
        Console.WriteLine(string.Join(",", numbers));
    }
}

A 1,3,4,5,2

B 1,2,3,4,5,4,5

C 1,3,4,5

D 1,3,4,5,4,5

💡 Hint

Remember the list is modified twice by the Action delegate and once by Remove.

Initially numbers = [1,2,3]. First addNumbers adds 4 and 5: [1,2,3,4,5]. Then Remove(2): [1,3,4,5]. Then addNumbers again adds 4 and 5: [1,3,4,5,4,5].