Delegate declaration and instantiation in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a delegate named MyDelegate that takes an int parameter.

C Sharp (C#)

public delegate [1] MyDelegate(int number);

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 method should not return anything.

The delegate declaration requires the return type first. Here, void means the delegate method returns nothing.

2fill in blank

medium

Complete the code to instantiate the delegate with a method named PrintNumber.

C Sharp (C#)

MyDelegate del = new MyDelegate([1]);

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

APrintNumber

BPrint

CShowNumber

DDisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist or does not match the delegate signature.

The delegate instance must be created with the exact method name that matches the delegate signature, here PrintNumber.

3fill in blank

hard

Fix the error in the delegate instantiation by completing the code.

C Sharp (C#)

MyDelegate del = [1] (PrintNumber);

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

Adelegate

Bnew delegate

Cnew MyDelegate

DMyDelegate

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the 'new' keyword or using incorrect keywords.

To instantiate a delegate, you must use the new MyDelegate keyword followed by the method name.

4fill in blank

hard

Fill both blanks to declare a delegate named Calculator that returns an int and takes two int parameters.

C Sharp (C#)

public delegate [1] Calculator([2] a, int b);

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

Aint

Bstring

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for parameters or return type that don't match the requirement.

The delegate return type is int and the first parameter type is also int.

5fill in blank

hard

Fill all three blanks to instantiate a delegate named Calculator with a method named Add and invoke it with arguments 5 and 10.

C Sharp (C#)

Calculator calc = [1]([2]);
int result = calc([3], 10);

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

Anew Calculator

BAdd

C5

DCalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting 'new' keyword, using wrong method name, or wrong argument values.

You create a new delegate instance with new Calculator, pass the method Add, and invoke it with the first argument 5 and second argument 10.

Practice

1. What is a delegate in C# used for?

easy

A. To define variables that cannot change

B. To hold references to methods with a specific signature

C. To store values like integers or strings

D. To create new classes dynamically

Solution

1. Step 1: Understand delegate purpose

   A delegate is a type that holds references to methods with a matching signature.

2. Step 2: Compare options to definition

   Only To hold references to methods with a specific signature correctly describes this purpose; others describe unrelated concepts.

3. Final Answer:

   To hold references to methods with a specific signature -> Option B

4. Quick Check:

   Delegate = method reference holder [OK]

Hint: Delegates point to methods, not data or classes [OK]

Common Mistakes:

• Confusing delegates with variables
• Thinking delegates create classes
• Assuming delegates store simple values

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

easy

A. void delegate MyDelegate(int number);

B. delegate int MyDelegate(void 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 exactly: delegate void MyDelegate(int number); others have incorrect order or types.

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' keyword + return type + name + parameters [OK]

Common Mistakes:

• Placing 'void' before 'delegate'
• Swapping delegate name and return type
• Using 'void' as parameter type

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

delegate int Operation(int x, int y);
class Program {
  static int Add(int a, int b) => a + b;
  static void Main() {
    Operation op = Add;
    Console.WriteLine(op(3, 4));
  }
}

medium

A. 0

B. 34

C. 7

D. Compile error

Solution

1. Step 1: Understand delegate assignment

   The delegate 'Operation' matches the method 'Add' signature (two ints, returns int), so 'op' points to 'Add'.

2. Step 2: Evaluate delegate invocation

   Calling 'op(3, 4)' calls 'Add(3, 4)', which returns 3 + 4 = 7.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   Delegate calls Add(3,4) = 7 [OK]

Hint: Delegate calls assigned method with given arguments [OK]

Common Mistakes:

• Thinking delegate concatenates numbers
• Expecting string output
• Assuming compile error due to delegate

4. Identify the error in the following delegate instantiation:

delegate void Printer(string message);
class Program {
  static void PrintMessage() { Console.WriteLine("Hello"); }
  static void Main() {
    Printer p = PrintMessage;
    p("Hi");
  }
}

medium

A. PrintMessage method signature does not match delegate

B. Delegate cannot be assigned to a method

C. Missing parentheses when assigning delegate

D. Delegate must return a value

Solution

1. Step 1: Compare delegate and method signatures

   The delegate 'Printer' expects a method with a string parameter and void return type.

2. Step 2: Check method signature

   'PrintMessage' has no parameters, so it does not match the delegate signature.

3. Final Answer:

   PrintMessage method signature does not match delegate -> Option A

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 needs parentheses on assignment
• Assuming delegate must return a value

5. Given the delegate declaration:

delegate bool Filter(int value);

Which of the following correctly instantiates the delegate to a method that returns true if the number is even?

hard

A. Filter f = x => x % 2 == 0;

B. Filter f = (int x) => { return x * 2; };

C. Filter f = delegate(int x) { Console.WriteLine(x); };

D. Filter f = x => x + 2 == 0;

Solution

1. Step 1: Understand delegate signature

   'Filter' expects a method taking an int and returning a bool.

2. Step 2: Check each option's return type and logic

   Filter f = x => x % 2 == 0; returns true if x is even (x % 2 == 0), matching bool return.
   Filter f = (int x) => { return x * 2; }; returns int, not bool.
   Filter f = delegate(int x) { Console.WriteLine(x); }; returns void (no return), invalid.
   Filter f = x => x + 2 == 0; returns bool but checks x + 2 == 0, which is not the even check.

3. Final Answer:

   Filter f = x => x % 2 == 0; -> Option A

4. Quick Check:

   Delegate needs bool return matching even check [OK]

Hint: Match delegate return type and logic exactly [OK]

Common Mistakes:

• Returning wrong type (int instead of bool)
• Missing return statement in anonymous method
• Incorrect condition for even check