Delegates as callback pattern in C Sharp (C#)

Introduction

Delegates let you pass methods as parameters so you can call them later. This helps you run different code when something happens, like a callback.

When you want to run a method after another method finishes.

When you want to customize behavior without changing the original method.

When you want to notify other parts of your program about an event.

When you want to pass different actions to a method to run later.

When you want to keep your code flexible and reusable.

Syntax

C Sharp (C#)

delegate ReturnType DelegateName(ParameterList);

// Example:
delegate void Callback(string message);

A delegate defines a type that can hold references to methods with a matching signature.

You can use delegates to call methods indirectly, enabling callback behavior.

Examples

Defines a delegate with no parameters and calls it inside another method.

C Sharp (C#)

delegate void SimpleCallback();

void Run(SimpleCallback callback) {
    callback();
}

Run(() => Console.WriteLine("Hello!"));

Delegate with a string parameter used to send a message back.

C Sharp (C#)

delegate void MessageCallback(string msg);

void ShowMessage(MessageCallback callback) {
    callback("Hi there!");
}

ShowMessage(msg => Console.WriteLine(msg));

Sample Program

This program shows how to use a delegate as a callback. The ProcessData method does some work and then calls the callback to notify about completion.

C Sharp (C#)

using System;

class Program {
    // Define a delegate type that takes a string and returns void
    delegate void Notify(string message);

    // Method that accepts a delegate as a callback
    static void ProcessData(Notify callback) {
        Console.WriteLine("Processing data...");
        // After processing, call the callback
        callback("Data processed successfully.");
    }

    static void Main() {
        // Call ProcessData and pass a method as callback
        ProcessData(PrintNotification);

        // Or use a lambda expression as callback
        ProcessData(msg => Console.WriteLine($"Lambda says: {msg}"));
    }

    static void PrintNotification(string message) {
        Console.WriteLine("Notification: " + message);
    }
}

Important Notes

Delegates can point to any method with the same signature, making callbacks flexible.

You can use lambda expressions to write inline callbacks without defining separate methods.

Delegates help keep your code clean by separating the action from the method doing the work.

Summary

Delegates let you pass methods as parameters to other methods.

This pattern is useful for callbacks to run code after some work is done.

You can use named methods or lambda expressions as callbacks.

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