Multicast delegates in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multicast delegate in C#?

A multicast delegate is a delegate that holds references to more than one method. When invoked, it calls all the methods it references in order.

beginner

How do you add methods to a multicast delegate?

You use the '+=' operator to add methods to a multicast delegate, combining them into a single delegate instance.

intermediate

What happens if one method in a multicast delegate throws an exception?

If a method in a multicast delegate throws an exception, the invocation stops and the exception is propagated. Methods after the exception are not called.

beginner

How can you remove a method from a multicast delegate?

You use the '-=' operator to remove a method from a multicast delegate, detaching it from the invocation list.

intermediate

What is the return value of a multicast delegate invocation?

For multicast delegates with a return type, only the return value of the last method called is returned.

Which operator is used to add a method to a multicast delegate in C#?

A *=

B -=

C +=

D /=

The '+=' operator adds a method to the invocation list of a multicast delegate.

What happens when you invoke a multicast delegate?

A Only the first method is called

B Only the last method is called

C No methods are called

D All methods in the invocation list are called in order

Invoking a multicast delegate calls all methods it references, in the order they were added.

If a method in a multicast delegate throws an exception, what happens?

A Invocation stops and the exception is thrown

B The exception is ignored

C All methods continue to run

D Only the last method runs

If any method throws an exception, the multicast delegate stops invoking further methods and propagates the exception.

How do you remove a method from a multicast delegate?

A +=

B -=

C *=

D /=

The '-=' operator removes a method from the multicast delegate's invocation list.

What return value does a multicast delegate provide when invoked?

A Return value of the last method

B Return values of all methods combined

C Return value of the first method

D No return value

Only the return value of the last method called by the multicast delegate is returned.

Explain what a multicast delegate is and how it works in C#.

Think about how one delegate can call many methods.

Describe how to add and remove methods from a multicast delegate and what happens when it is invoked.

Focus on the operators and the invocation process.

Practice

1. What is the main purpose of a multicast delegate in C#?

easy

A. To call multiple methods with a single delegate invocation

B. To store only one method reference at a time

C. To create a delegate that returns multiple values

D. To prevent methods from being called more than once

Solution

1. Step 1: Understand delegate basics

   A delegate holds references to methods and can invoke them.

2. Step 2: Identify multicast delegate behavior

   Multicast delegates can hold multiple method references and call them all in order.

3. Final Answer:

   To call multiple methods with a single delegate invocation -> Option A

4. Quick Check:

   Multicast delegates call many methods [OK]

Hint: Multicast delegates call many methods at once [OK]

Common Mistakes:

• Thinking multicast delegates return multiple values
• Confusing with single method delegates
• Assuming multicast delegates prevent repeated calls

2. Which syntax correctly adds a method named ShowMessage to a multicast delegate del?

easy

A. del += ShowMessage;

B. del -+ ShowMessage;

C. del =+ ShowMessage;

D. del = ShowMessage;

Solution

1. Step 1: Recall delegate addition syntax

   To add a method to a multicast delegate, use the '+=' operator.

2. Step 2: Check each option

   del += ShowMessage; uses 'del += ShowMessage;' which is correct syntax to add a method.

3. Final Answer:

   del += ShowMessage; -> Option A

4. Quick Check:

   Use '+=' to add methods to multicast delegates [OK]

Hint: Use '+=' to add methods to multicast delegates [OK]

Common Mistakes:

• Using '=' instead of '+=' to add methods
• Typing operators incorrectly like '=+' or '-+'
• Confusing removal syntax with addition

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

delegate void Notify();
class Program {
  static void A() => Console.WriteLine("A called");
  static void B() => Console.WriteLine("B called");
  static void Main() {
    Notify del = A;
    del += B;
    del();
  }
}

medium

A. No output

B. B called
A called

C. A called

D. A called
B called

Solution

1. Step 1: Understand delegate invocation order

   Methods added to a multicast delegate are called in the order they were added.

2. Step 2: Trace the code execution

   First, method A is assigned, then method B is added. Calling del() invokes A then B.

3. Final Answer:

   A called B called -> Option D

4. Quick Check:

   Multicast delegates call methods in added order [OK]

Hint: Methods run in order added to delegate [OK]

Common Mistakes:

• Assuming reverse order of method calls
• Expecting only the last method to run
• Thinking no output occurs without explicit loop

4. Identify the error in this code snippet:

delegate void Notify();
class Program {
  static void A() => Console.WriteLine("A called");
  static void Main() {
    Notify del = A;
    del -= A;
    del();
  }
}

medium

A. Syntax error in delegate declaration

B. del is null when invoked, causing a runtime error

C. Cannot remove method from delegate

D. No error, output is 'A called'

Solution

1. Step 1: Analyze delegate removal

   Removing the only method from the delegate leaves it null.

2. Step 2: Check invocation after removal

   Calling del() when del is null causes a NullReferenceException at runtime.

3. Final Answer:

   del is null when invoked, causing a runtime error -> Option B

4. Quick Check:

   Removing last method makes delegate null [OK]

Hint: Removing last method makes delegate null, check before calling [OK]

Common Mistakes:

• Assuming delegate still holds method after removal
• Ignoring null check before invoking delegate
• Thinking removal causes syntax error

5. You want to create a multicast delegate that calls three methods: LogStart, ProcessData, and LogEnd. Which code correctly sets up and invokes this delegate?

hard

A. Notify del; del = LogStart; del = ProcessData; del = LogEnd; del();

B. Notify del = LogStart + ProcessData + LogEnd; del();

C. Notify del = LogStart; del += ProcessData; del += LogEnd; del();

D. Notify del = LogStart; del -= ProcessData; del += LogEnd; del();

Solution

1. Step 1: Understand delegate combination

   Use '+=' to add methods to a multicast delegate one by one.

2. Step 2: Evaluate each option

   Notify del = LogStart; del += ProcessData; del += LogEnd; del(); correctly initializes with LogStart, then adds ProcessData and LogEnd, then calls del().

3. Final Answer:

   Notify del = LogStart; del += ProcessData; del += LogEnd; del(); -> Option C

4. Quick Check:

   Use '+=' repeatedly to build multicast delegate [OK]

Hint: Add methods one by one with '+=' to build multicast delegate [OK]

Common Mistakes:

• Using '+' operator directly without delegate type
• Overwriting delegate instead of adding methods
• Removing methods unintentionally before calling