Delegate declaration and instantiation in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a delegate in C#?

A delegate is a type that represents references to methods with a specific parameter list and return type. It allows methods to be passed as parameters or assigned to variables.

beginner

How do you declare a delegate that takes an int and returns void?

You declare it like this:

public delegate void MyDelegate(int number);

beginner

How do you instantiate a delegate to point to a method named 'PrintNumber'?

You instantiate it like this:

MyDelegate d = new MyDelegate(PrintNumber);

or simply

MyDelegate d = PrintNumber;

intermediate

Can a delegate hold references to multiple methods?

Yes, delegates can hold references to multiple methods. This is called a multicast delegate, and it calls all methods in its invocation list when invoked.

beginner

What happens when you invoke a delegate?

When you invoke a delegate, it calls the method(s) it references with the provided arguments, and returns the result if the method has a return type.

How do you declare a delegate that returns an int and takes no parameters?

A delegate void MyDelegate(int);

B public int delegate MyDelegate();

C public delegate int MyDelegate();

D delegate MyDelegate int();

The correct syntax is 'public delegate int MyDelegate();' which declares a delegate returning int and taking no parameters.

Which of the following is a valid way to instantiate a delegate named 'MyDelegate' pointing to method 'Show'?

A MyDelegate d = new MyDelegate(Show);

B MyDelegate d = Show();

C MyDelegate d = delegate Show();

D MyDelegate d = new Show();

You instantiate a delegate by using 'new MyDelegate(MethodName);' or simply 'MyDelegate d = MethodName;'.

What is a multicast delegate?

A A delegate that can reference multiple methods.

B A delegate that returns multiple values.

C A delegate that can only reference one method.

D A delegate that is declared inside a class.

A multicast delegate holds references to multiple methods and calls them all when invoked.

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

A action

B event

C func

D delegate

The 'delegate' keyword is used to declare a delegate type.

If a delegate references multiple methods, what happens when you invoke it?

A Only the last method is called.

B All referenced methods are called in order.

C Only the first method is called.

D An error occurs.

A multicast delegate calls all methods in its invocation list in order when invoked.

Explain how to declare and instantiate a delegate in C# with an example.

Think about how you define a delegate type and then create an object of that type pointing to a method.

Describe what happens when you invoke a multicast delegate.

Consider how a delegate can hold more than one method and what that means when you call it.

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