Delegates as callback pattern in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a delegate named Callback that takes no parameters and returns void.

C Sharp (C#)

public delegate [1] Callback();

Drag options to blanks, or click blank then click option'

Astring

Bvoid

Cint

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using a return type other than void when the callback does not return a value.

The delegate must return void because it represents a callback with no return value.

2fill in blank

medium

Complete the code to declare a method named Process that accepts a Callback delegate as a parameter.

C Sharp (C#)

public void Process([1] callback) { callback(); }

Drag options to blanks, or click blank then click option'

Aint

Bstring

CCallback

DAction

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type like int or string instead of the delegate type.

The method parameter must be of type Callback to accept the delegate.

3fill in blank

hard

Fix the error in the code by completing the delegate invocation inside the Process method.

C Sharp (C#)

public void Process(Callback callback) { callback[1]; }

Drag options to blanks, or click blank then click option'

A()

B;

C{}

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add parentheses when calling the delegate.

Delegates are invoked like methods, so parentheses () are needed to call the delegate.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps strings to their lengths only if length is greater than 3.

C Sharp (C#)

var lengths = new Dictionary<string, int> { { [1], [2] } };

Drag options to blanks, or click blank then click option'

A"apple"

B"banana"

C"pear"

D"kiwi"

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid syntax for dictionary initialization.

This task is a trick question to check understanding of dictionary initialization syntax. The correct answer is to fill the blanks with key and value strings.

5fill in blank

hard

Fill all three blanks to define and use a delegate as a callback that prints a message.

C Sharp (C#)

public delegate void [1]();

public void Run([2] callback) {
    callback[3];
}

Drag options to blanks, or click blank then click option'

ACallback

C()

DAction

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for delegate and parameter.

Forgetting parentheses when invoking the delegate.

The delegate is named Callback, the parameter type is Callback, and the delegate is invoked with parentheses ().

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