Delegates as callback pattern in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a delegate in C#?

A delegate is a type that holds references to methods with a specific signature. It allows methods to be passed as parameters or assigned to variables.

beginner

How does a delegate enable the callback pattern?

A delegate lets you pass a method as an argument to another method. The receiving method can then call the passed method later, acting as a callback.

intermediate

Show a simple example of a delegate used as a callback.

public delegate void Notify(string message);

public class Process {
    public void Start(Notify callback) {
        // Do some work
        callback("Process finished");
    }
}

// Usage
Process p = new Process();
p.Start(msg => Console.WriteLine(msg));

intermediate

What is the benefit of using delegates for callbacks?

Delegates provide flexibility by allowing different methods to be called without changing the caller code. This supports loose coupling and reusable code.

advanced

Can delegates point to multiple methods? What is this called?

Yes, delegates can point to multiple methods. This is called a multicast delegate. When invoked, all methods are called in order.

What does a delegate in C# represent?

A A class that inherits from another class

B A variable that stores data

C A type of loop

D A reference to a method with a specific signature

A delegate holds references to methods matching its signature, enabling callbacks.

How do delegates support the callback pattern?

A By creating new classes dynamically

B By storing data in arrays

C By allowing methods to be passed as parameters and called later

D By running code in parallel automatically

Delegates let you pass methods as arguments, which can be called back later.

What keyword is used to declare a delegate in C#?

A function

B delegate

C callback

D event

The 'delegate' keyword declares a delegate type.

What happens when a multicast delegate is invoked?

A All methods assigned to it are called in order

B Only the first method is called

C It throws an error

D It returns a list of methods

Multicast delegates call all their assigned methods sequentially.

Which of these is a benefit of using delegates for callbacks?

A They allow loose coupling between caller and callee

B They increase program size significantly

C They prevent any method from being called

D They make code run slower always

Delegates enable loose coupling by letting different methods be called without changing the caller.

Explain how delegates enable the callback pattern in C#.

Think about how you can give a method to another method to call back.

Describe the difference between a single-cast and multicast delegate.

Consider how many methods a delegate can point to.

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