Multicast delegates in C Sharp (C#) - Mini Project: Build & Apply

Multicast delegates

📖 Scenario: Imagine you are building a simple notification system where multiple actions happen when an event occurs, like sending an email and logging a message.

🎯 Goal: You will create a multicast delegate that calls multiple methods when invoked, demonstrating how multicast delegates work in C#.

📋 What You'll Learn

Create a delegate type named Notify that takes no parameters and returns void

Create two methods named SendEmail and LogMessage that print specific messages

Create a multicast delegate instance combining SendEmail and LogMessage

Invoke the multicast delegate to see both messages printed

💡 Why This Matters

🌍 Real World

Multicast delegates are useful in event handling systems where multiple actions need to respond to a single event, such as UI updates, logging, and notifications.

💼 Career

Understanding multicast delegates helps in building responsive and modular C# applications, especially in desktop, web, and game development.

1

Create the delegate type and methods

Create a delegate type called Notify that takes no parameters and returns void. Then create two methods: SendEmail that prints "Email sent." and LogMessage that prints "Message logged."

C Sharp (C#)

// Create delegate type Notify
// Create method SendEmail that prints "Email sent."
// Create method LogMessage that prints "Message logged."

Hint

Use delegate void Notify() to declare the delegate type. Methods must match this signature.

2

Create a multicast delegate instance

Inside the Main method, create a variable called notifyHandlers of type Notify and assign it to SendEmail. Then add LogMessage to notifyHandlers using the += operator.

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    public static void SendEmail()
    {
        Console.WriteLine("Email sent.");
    }

    public static void LogMessage()
    {
        Console.WriteLine("Message logged.");
    }

    static void Main()
    {
        // Create Notify variable notifyHandlers and assign SendEmail
        // Add LogMessage to notifyHandlers
    }
}

Hint

Use Notify notifyHandlers = SendEmail; and then notifyHandlers += LogMessage; to combine methods.

3

Invoke the multicast delegate

Still inside the Main method, invoke the notifyHandlers delegate to call both SendEmail and LogMessage methods.

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    public static void SendEmail()
    {
        Console.WriteLine("Email sent.");
    }

    public static void LogMessage()
    {
        Console.WriteLine("Message logged.");
    }

    static void Main()
    {
        Notify notifyHandlers = SendEmail;
        notifyHandlers += LogMessage;
        // Invoke notifyHandlers
    }
}

Hint

Call the delegate like a method: notifyHandlers();

4

Display the output

Run the program and observe the output. It should print both "Email sent." and "Message logged." lines.

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    public static void SendEmail()
    {
        Console.WriteLine("Email sent.");
    }

    public static void LogMessage()
    {
        Console.WriteLine("Message logged.");
    }

    static void Main()
    {
        Notify notifyHandlers = SendEmail;
        notifyHandlers += LogMessage;
        notifyHandlers();
        // Output is shown automatically
    }
}

Hint

Both messages should appear one after the other when you run the program.

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