Multicast delegates in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a delegate named MyDelegate.

C Sharp (C#)

public delegate void [1]();

Drag options to blanks, or click blank then click option'

AMyDelegate

BAction

CDelegate

DFunc

Attempts:

3 left

💡 Hint

Common Mistakes

Using built-in delegate types like Action or Func instead of declaring a new delegate.

Using an invalid delegate name.

The delegate is declared with the name MyDelegate to match the usage later.

2fill in blank

medium

Complete the code to add method1 to the multicast delegate.

C Sharp (C#)

MyDelegate d = method1; d [1]= method2;

Drag options to blanks, or click blank then click option'

A*=

B+

C+=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of +=.

Using -= which removes a method.

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

3fill in blank

hard

Fix the error in the code to invoke the multicast delegate safely.

C Sharp (C#)

if ([1] != null) d();

Drag options to blanks, or click blank then click option'

Amethod1

Bd

CMyDelegate

Ddelegate

Attempts:

3 left

💡 Hint

Common Mistakes

Checking a method name instead of the delegate variable.

Not checking for null before invoking.

Before invoking a delegate, check if the delegate variable d is not null to avoid exceptions.

4fill in blank

hard

Fill both blanks to remove method2 from the multicast delegate and invoke it.

C Sharp (C#)

d [1]= method2; if (d [2] null) d();

Drag options to blanks, or click blank then click option'

A-=

B!=

C==

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using += instead of -= to remove a method.

Checking d == null instead of d != null.

Use -= to remove a method from the delegate. Then check if d != null before invoking.

5fill in blank

hard

Fill all three blanks to create a multicast delegate, add two methods, and invoke it safely.

C Sharp (C#)

MyDelegate d = method1; d [1]= method2; if (d [2] null) d[3]();

Drag options to blanks, or click blank then click option'

A+=

B!=

C()

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add methods with +=.

Not checking for null before invoking.

Missing parentheses when calling the delegate.

Use += to add methods, check d != null before invoking, and call the delegate with ().

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