Delegate declaration and instantiation in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of delegate invocation

What is the output of this C# code when the delegate is invoked?

C Sharp (C#)

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));
    }
}

A 7

B 34

C 0

D Compilation error

💡 Hint

Remember that the delegate points to the Add method which sums two integers.

The delegate 'Operation' is assigned the method 'Add'. Calling op(3,4) invokes Add(3,4), which returns 7.

❓ Predict Output

intermediate

Delegate with anonymous method output

What will be printed when this C# program runs?

C Sharp (C#)

delegate string Formatter(string s);

class Program {
    static void Main() {
        Formatter f = delegate(string input) { return input.ToUpper(); };
        Console.WriteLine(f("hello"));
    }
}

A HELLO

B hello

C Hello

D Compilation error

💡 Hint

The anonymous method converts the string to uppercase.

The delegate 'Formatter' is assigned an anonymous method that returns the uppercase version of the input string. So 'hello' becomes 'HELLO'.

🔧 Debug

advanced

Identify the error in delegate instantiation

Which option correctly fixes the error in this delegate instantiation code?

C Sharp (C#)

delegate void Printer(string message);

class Program {
    static void PrintMessage() {
        Console.WriteLine("Hello World");
    }
    static void Main() {
        Printer p = PrintMessage;
        p("Test");
    }
}

A Change delegate return type to string: delegate string Printer(string message);

B Remove the parameter from delegate Printer: delegate void Printer();

C Call p() without arguments: p();

D Change PrintMessage to accept a string parameter: static void PrintMessage(string msg)

💡 Hint

The delegate expects a method with one string parameter, but PrintMessage has none.

The delegate 'Printer' requires a method that takes a string parameter. The method 'PrintMessage' has no parameters, so it must be changed to accept a string.

❓ Predict Output

advanced

Multicast delegate invocation output

What is the output of this C# program?

C Sharp (C#)

delegate void Notify();

class Program {
    static void Alert1() => Console.WriteLine("Alert 1");
    static void Alert2() => Console.WriteLine("Alert 2");
    static void Main() {
        Notify n = Alert1;
        n += Alert2;
        n();
    }
}

A Alert 2\nAlert 1

B Alert 1\nAlert 2

C Alert 1

D Compilation error

💡 Hint

Multicast delegates call all methods in the order they were added.

The delegate 'n' calls Alert1 first, then Alert2, so both messages print in that order.

🧠 Conceptual

expert

Delegate instantiation with lambda expressions

Which option correctly declares and instantiates a delegate that takes two integers and returns their product using a lambda expression?

A Func multiply = (int x, int y) => x * y;

B delegate int multiply(int x, int y) => x * y;

C Func<int, int, int> multiply = (x, y) => x * y;

D var multiply = delegate(int x, int y) { return x + y; };

💡 Hint

Use Func<> for delegates with return values and lambda syntax for instantiation.

Option C correctly uses Func delegate type and a lambda expression returning the product. Option C is invalid syntax. Option C misses generic parameters. Option C returns sum, not product.

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