Multicast delegates in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Multicast delegates

Create delegate instance

↓

Add method1 to delegate

↓

Add method2 to delegate

↓

Invoke delegate

↓

Call method1

↓

Call method2

↓

End

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

Execution Sample

C Sharp (C#)

delegate void Notify();

void Method1() => Console.WriteLine("Method1 called");
void Method2() => Console.WriteLine("Method2 called");

Notify notify = Method1;
notify += Method2;
notify();

This code creates a multicast delegate that calls Method1 and Method2 in sequence when invoked.

Execution Table

Step	Action	Delegate State	Output
1	Create delegate notify with Method1	notify -> [Method1]
2	Add Method2 to notify	notify -> [Method1, Method2]
3	Invoke notify()	Calls Method1 then Method2	Method1 called
4	Method2 called after Method1	All methods called in order	Method2 called
5	End of delegate invocation	No more methods to call

💡 All methods in the multicast delegate have been called in order.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
notify	null	[Method1]	[Method1, Method2]	[Invoked]	[Invoked]

Key Moments - 3 Insights

Why does invoking the delegate call both Method1 and Method2?

Because the delegate holds references to both methods (see execution_table rows 2 and 3), invoking it calls each method in the order they were added.

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

The invocation stops at that method and later methods are not called. This is important to handle exceptions carefully when using multicast delegates.

Can multicast delegates return values?

Multicast delegates can only return the value of the last method called. Earlier return values are ignored.

Visual Quiz - 3 Questions

Look at the execution_table, what is the delegate state after Step 2?

A notify -> [Method1, Method2]

B notify -> [Method2]

C notify -> [Method1]

D notify is null

💡 Hint

Check the 'Delegate State' column in row 2 of the execution_table.

At which step does Method2 get called?

A Step 1

B Step 4

C Step 3

D Step 2

💡 Hint

Look at the 'Output' column in the execution_table rows 3 and 4.

If we remove Method1 from the delegate, what would be the output at Step 3?

A No output

B Method1 called

C Method2 called

D Both Method1 and Method2 called

💡 Hint

Refer to the delegate state and output in the execution_table for how methods are called.

Concept Snapshot

Multicast delegates hold multiple method references.
Use += to add methods.
Invoke calls all methods in order.
Return value is from last method only.
Exceptions stop further calls.

Full Transcript

This visual trace shows how multicast delegates work in C#. First, a delegate instance is created pointing to Method1. Then Method2 is added to the delegate using +=. When the delegate is invoked, it calls Method1 first, then Method2. The variable 'notify' changes from null to holding Method1, then both Method1 and Method2. The output shows the calls in order. Key points include that all methods are called in sequence, exceptions stop the chain, and only the last method's return value is used.

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