Action delegate type in C Sharp (C#) - Mini Project: Build & Apply

Using the Action Delegate Type in C#

📖 Scenario: You are creating a simple program to greet users and show messages using the Action delegate type in C#. This helps you run small pieces of code stored in variables.

🎯 Goal: Build a program that stores greeting and farewell messages in Action delegates and runs them to display messages on the screen.

📋 What You'll Learn

Create an Action delegate variable called greet that prints a greeting message.

Create another Action delegate variable called farewell that prints a farewell message.

Invoke both greet and farewell to show the messages.

💡 Why This Matters

🌍 Real World

Using <code>Action</code> delegates helps you store and run small tasks or callbacks, like button clicks or timed events, in real programs.

💼 Career

Understanding delegates is important for event-driven programming and working with frameworks like Windows Forms, WPF, or ASP.NET.

1

Create an Action delegate called greet

Write a line of code to create an Action delegate variable named greet that prints "Hello, welcome!" when called.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        // Create an Action delegate called greet that prints "Hello, welcome!"
        
    }
}

Hint

Use Action with a lambda expression that calls Console.WriteLine.

2

Create an Action delegate called farewell

Add a line of code to create an Action delegate variable named farewell that prints "Goodbye, see you soon!" when called.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        Action greet = () => Console.WriteLine("Hello, welcome!");
        // Create an Action delegate called farewell that prints "Goodbye, see you soon!"
        
    }
}

Hint

Use the same pattern as greet but with the farewell message.

3

Invoke the greet and farewell delegates

Write code to call the greet and farewell delegates to display their messages.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        Action greet = () => Console.WriteLine("Hello, welcome!");
        Action farewell = () => Console.WriteLine("Goodbye, see you soon!");
        // Call greet and farewell to show messages
        
    }
}

Hint

Call each delegate by adding parentheses after their names.

4

Display the output

Run the program and write a line of code to print the output of calling greet and farewell delegates.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        Action greet = () => Console.WriteLine("Hello, welcome!");
        Action farewell = () => Console.WriteLine("Goodbye, see you soon!");
        greet();
        farewell();
        // Output is shown by the calls above
    }
}

Hint

The output appears automatically when you run the program because of the Console.WriteLine calls inside the delegates.

Practice

1. What is the primary purpose of the Action delegate type in C#?

easy

A. To represent methods that return void and can take parameters

B. To represent methods that return a value

C. To represent asynchronous methods only

D. To represent methods that take no parameters and return a value

Solution

1. Step 1: Understand what Action represents

   Action is a delegate type used to represent methods that do not return a value (void) but can take parameters.

2. Step 2: Compare options with the definition

   To represent methods that return void and can take parameters correctly states that Action represents methods returning void and can take parameters. Other options mention return values or asynchronous behavior, which are incorrect.

3. Final Answer:

   To represent methods that return void and can take parameters -> Option A

4. Quick Check:

   Action = void methods with parameters [OK]

Hint: Action always means void return with optional parameters [OK]

Common Mistakes:

• Confusing Action with Func which returns a value
• Thinking Action is only for methods without parameters
• Assuming Action is for async methods only

2. Which of the following is the correct syntax to declare an Action delegate that takes two integer parameters?

easy

A. Action myAction(int, int);

B. Action<int> myAction;

C. Action myAction;

D. Action myAction() => void;

Solution

1. Step 1: Recall Action delegate syntax

   The Action delegate syntax for parameters is Action<T1, T2> for two parameters.

2. Step 2: Analyze each option

   Action myAction; correctly declares an Action with two int parameters. Action<int> myAction; has only one parameter. Action myAction(int, int); is invalid syntax. Action myAction() => void; is incorrect syntax and return type usage.

3. Final Answer:

   Action myAction; -> Option C

4. Quick Check:

   Two parameters use Action<int, int> [OK]

Hint: Use Action for two parameters [OK]

Common Mistakes:

• Using parentheses like a method instead of generic syntax
• Declaring return type with Action (it always returns void)
• Confusing Action with Func syntax

3. What will be the output of the following C# code?

Action<string> greet = name => Console.WriteLine($"Hello, {name}!");
greet("Alice");

medium

A. Hello, name!

B. Hello, Alice!

C. greet

D. No output

Solution

1. Step 1: Understand the Action delegate usage

   The greet variable is an Action<string> delegate that takes a string and prints a greeting with that string.

2. Step 2: Trace the method call

   Calling greet("Alice") runs the lambda, printing "Hello, Alice!" to the console.

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   Action<string> prints Hello, Alice! [OK]

Hint: Action runs the method body with given parameters [OK]

Common Mistakes:

• Expecting variable name printed instead of parameter value
• Thinking Action returns a value to print
• Missing that Console.WriteLine outputs text

4. Identify the error in the following code snippet:

Action<int> printNumber = (int num) => return num.ToString();

medium

A. Lambda expression cannot use return with Action delegate

B. Action delegate cannot have parameters

C. Missing semicolon after lambda

D. Action delegate must return a string

Solution

1. Step 1: Understand Action delegate return type

   Action delegates always return void, so their lambda expressions cannot return a value.

2. Step 2: Analyze the lambda expression

   The lambda tries to return num.ToString(), which is a string, causing a mismatch with void return type.

3. Final Answer:

   Lambda expression cannot use return with Action delegate -> Option A

4. Quick Check:

   Action = void, no return allowed [OK]

Hint: Action lambdas must not return values [OK]

Common Mistakes:

• Trying to return a value from an Action delegate
• Confusing Action with Func delegate
• Ignoring lambda syntax rules

5. You want to create an Action delegate that takes three parameters: an int, a string, and a bool. It should print a message only if the bool is true. Which of the following code snippets correctly implements this?

hard

A. Action printIfTrue = (num, text) => { if(flag) Console.WriteLine($"{num}: {text}"); };

B. Action printIfTrue = (num, text, flag) => { if(flag) return Console.WriteLine($"{num}: {text}"); };

C. Action printIfTrue = (num, text, flag) => Console.WriteLine($"{num}: {text}");

D. Action printIfTrue = (num, text, flag) => { if(flag) Console.WriteLine($"{num}: {text}"); };

Solution

1. Step 1: Check parameter count and types

   Action printIfTrue = (num, text, flag) => { if(flag) Console.WriteLine($"{num}: {text}"); }; correctly declares an Action with three parameters: int, string, and bool. Action printIfTrue = (num, text) => { if(flag) Console.WriteLine($"{num}: {text}"); }; has only two parameters, so it's invalid.

2. Step 2: Verify the lambda body logic

   Action printIfTrue = (num, text, flag) => { if(flag) Console.WriteLine($"{num}: {text}"); }; uses an if statement to print only if the bool is true, which matches the requirement. Action printIfTrue = (num, text, flag) => { if(flag) return Console.WriteLine($"{num}: {text}"); }; incorrectly uses return with Console.WriteLine, which is invalid for Action. Action printIfTrue = (num, text, flag) => Console.WriteLine($"{num}: {text}"); always prints regardless of the bool.

3. Final Answer:

   Action printIfTrue = (num, text, flag) => { if(flag) Console.WriteLine($"{num}: {text}"); }; -> Option D

4. Quick Check:

   Correct params and conditional print = Action printIfTrue = (num, text, flag) => { if(flag) Console.WriteLine($"{num}: {text}"); }; [OK]

Hint: Match parameters and avoid return in Action lambdas [OK]

Common Mistakes:

• Using return inside Action lambda
• Mismatching parameter count
• Ignoring the bool condition in printing