Delegates as callback pattern in C Sharp (C#) - Mini Project: Build & Apply

Delegates as callback pattern

📖 Scenario: You are building a simple notification system where a process can notify different actions when it finishes.

🎯 Goal: Create a delegate to act as a callback, assign methods to it, and invoke it to notify when a process is done.

📋 What You'll Learn

Create a delegate type called ProcessCompleted that takes no parameters and returns void

Create a method called NotifyUser that prints "Process completed!"

Create a method called LogCompletion that prints "Logging: Process finished."

Create a delegate variable called callback and assign NotifyUser to it

Add LogCompletion to the callback delegate invocation list

Invoke the callback delegate to run all assigned methods

💡 Why This Matters

🌍 Real World

Delegates are used to implement callbacks, event handling, and flexible method calls in many C# applications like UI programming and asynchronous operations.

💼 Career

Understanding delegates is essential for C# developers to write modular, reusable, and event-driven code in professional software development.

1

Create the delegate type and methods

Create a delegate type called ProcessCompleted that takes no parameters and returns void. Then create two methods: NotifyUser that prints "Process completed!" and LogCompletion that prints "Logging: Process finished."

C Sharp (C#)

// Your code here

Hint

Use public delegate void ProcessCompleted() to declare the delegate. Define two static methods with Console.WriteLine inside.

2

Create the delegate variable and assign the first method

Inside Main, create a variable called callback of type ProcessCompleted and assign it to the method NotifyUser.

C Sharp (C#)

using System;

public delegate void ProcessCompleted();

class Program
{
    public static void NotifyUser()
    {
        Console.WriteLine("Process completed!");
    }

    public static void LogCompletion()
    {
        Console.WriteLine("Logging: Process finished.");
    }

    static void Main()
    {
        // Create delegate variable callback and assign NotifyUser
        // Your code here
    }
}

Hint

Use ProcessCompleted callback = NotifyUser; to assign the method to the delegate variable.

3

Add the second method to the delegate

Add the method LogCompletion to the callback delegate invocation list using the += operator.

C Sharp (C#)

using System;

public delegate void ProcessCompleted();

class Program
{
    public static void NotifyUser()
    {
        Console.WriteLine("Process completed!");
    }

    public static void LogCompletion()
    {
        Console.WriteLine("Logging: Process finished.");
    }

    static void Main()
    {
        ProcessCompleted callback = NotifyUser;
        // Add LogCompletion to callback
        // Your code here
    }
}

Hint

Use callback += LogCompletion; to add the method to the delegate.

4

Invoke the delegate to run all methods

Invoke the callback delegate to run all assigned methods.

C Sharp (C#)

using System;

public delegate void ProcessCompleted();

class Program
{
    public static void NotifyUser()
    {
        Console.WriteLine("Process completed!");
    }

    public static void LogCompletion()
    {
        Console.WriteLine("Logging: Process finished.");
    }

    static void Main()
    {
        ProcessCompleted callback = NotifyUser;
        callback += LogCompletion;
        // Invoke the callback delegate
        // Your code here
    }
}

Hint

Call the delegate like a method: callback();

Practice

1. What is the main purpose of using delegates as a callback pattern in C#?

easy

A. To handle exceptions automatically

B. To create new classes dynamically

C. To pass methods as parameters and execute them later

D. To convert data types implicitly

Solution

1. Step 1: Understand delegate usage

   Delegates allow methods to be passed as parameters to other methods.

2. Step 2: Identify callback purpose

   This passing enables callbacks, which run code after some work is done.

3. Final Answer:

   To pass methods as parameters and execute them later -> Option C

4. Quick Check:

   Delegates = method parameters for callbacks [OK]

Hint: Delegates let you pass methods to run later [OK]

Common Mistakes:

• Confusing delegates with classes
• Thinking delegates handle exceptions
• Assuming delegates convert data types

2. Which of the following is the correct syntax to declare a delegate that takes an int and returns void?

easy

A. MyDelegate delegate void(int number);

B. void delegate MyDelegate(int number);

C. delegate MyDelegate void(int number);

D. delegate void MyDelegate(int number);

Solution

1. Step 1: Recall delegate declaration syntax

   The correct syntax is: delegate returnType DelegateName(parameterList);

2. Step 2: Match syntax to options

   delegate void MyDelegate(int number); matches this pattern: delegate void MyDelegate(int number);

3. Final Answer:

   delegate void MyDelegate(int number); -> Option D

4. Quick Check:

   delegate returnType Name(params) = delegate void MyDelegate(int number); [OK]

Hint: Remember: delegate returnType Name(params) [OK]

Common Mistakes:

• Placing 'void' before 'delegate'
• Swapping delegate and return type order
• Misplacing delegate keyword after name

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

delegate void Callback(string message);  
class Program {
  static void Main() {
    Callback cb = msg => Console.WriteLine($"Hello, {msg}!");
    RunProcess(cb);
  }
  static void RunProcess(Callback callback) {
    Console.WriteLine("Starting process...");
    callback("World");
    Console.WriteLine("Process finished.");
  }
}

medium

A. Starting process...
Hello, World!
Process finished.

B. Hello, World!
Starting process...
Process finished.

C. Starting process...
Process finished.
Hello, World!

D. Process finished.
Starting process...
Hello, World!

Solution

1. Step 1: Trace method calls in Main and RunProcess

   Main creates a callback that prints "Hello, {msg}!" and passes it to RunProcess. RunProcess prints "Starting process...", calls callback with "World", then prints "Process finished.".

2. Step 2: Determine output order

   Output lines are: "Starting process...", "Hello, World!", "Process finished." in that order.

3. Final Answer:

   Starting process... Hello, World! Process finished. -> Option A

4. Quick Check:

   Callback runs between start and finish prints [OK]

Hint: Callbacks run where invoked inside method [OK]

Common Mistakes:

• Assuming callback runs before 'Starting process...'
• Mixing output order
• Ignoring lambda execution timing

4. Identify the error in this code snippet using delegates as callbacks:

delegate void Notify();
class Test {
  static void Main() {
    Notify n = NotifyMethod;
    Run(n);
  }
  static void Run(Notify callback) {
    callback();
  }
  static void NotifyMethod(string message) {
    Console.WriteLine(message);
  }
}

medium

A. Delegate is not instantiated correctly

B. Delegate signature does not match method signature

C. Callback method is not called

D. Missing return statement in NotifyMethod

Solution

1. Step 1: Compare delegate and method signatures

   Delegate Notify expects a method with no parameters and void return. NotifyMethod takes a string parameter, so signatures differ.

2. Step 2: Understand delegate assignment rules

   Assigning NotifyMethod to Notify delegate causes a compile-time error due to signature mismatch.

3. Final Answer:

   Delegate signature does not match method signature -> Option B

4. Quick Check:

   Delegate and method signatures must match [OK]

Hint: Delegate and method signatures must match exactly [OK]

Common Mistakes:

• Ignoring parameter mismatch
• Thinking delegate instantiation is wrong
• Assuming missing return causes error here

5. You want to create a method ProcessData that takes an integer array and a delegate callback to process each element asynchronously. Which approach correctly uses delegates as callbacks to print each element after processing?

hard

A. Define a delegate Action<int> and call it inside a loop with Task.Run(() => callback(item))

B. Use a delegate with no parameters and call it once after processing all elements

C. Pass a delegate that returns int and ignore its return value

D. Use a delegate that takes a string but pass integers directly

Solution

1. Step 1: Identify delegate type for element processing

   Since each element is an int and we want to process it, Action<int> fits as it takes int and returns void.

2. Step 2: Use asynchronous calls inside loop

   Calling Task.Run(() => callback(item)) runs the callback asynchronously for each element.

3. Final Answer:

   Define a delegate Action<int> and call it inside a loop with Task.Run(() => callback(item)) -> Option A

4. Quick Check:

   Delegate matches parameter and async call runs callback per item [OK]

Hint: Use Action<int> and async calls inside loop [OK]

Common Mistakes:

• Using delegate with wrong parameters
• Calling callback only once instead of per item
• Ignoring async execution