Multicast delegates in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Multicast Delegate Invocation

What is the output of the following C# code using multicast delegates?

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    static void Main()
    {
        Notify notify = MethodA;
        notify += MethodB;
        notify();
    }

    static void MethodA() => Console.WriteLine("MethodA called");
    static void MethodB() => Console.WriteLine("MethodB called");
}

A Compilation error

B MethodB called\nMethodA called

C MethodA called\nMethodB called

D MethodA called

💡 Hint

Multicast delegates invoke methods in the order they were added.

When invoking a multicast delegate, all methods are called in the order they were added. Here, MethodA is added first, then MethodB, so MethodA runs before MethodB.

❓ Predict Output

intermediate

Return value of multicast delegate with int return type

What is the output of this C# code that uses a multicast delegate with an int return type?

C Sharp (C#)

using System;

public delegate int Compute(int x);

class Program
{
    static void Main()
    {
        Compute compute = Square;
        compute += Double;
        int result = compute(3);
        Console.WriteLine(result);
    }

    static int Square(int x) => x * x;
    static int Double(int x) => x * 2;
}

A 6

B 9

C 15

D Compilation error

💡 Hint

Multicast delegates with return values return the last method's result.

When a multicast delegate has a return type, only the return value of the last invoked method is returned. Here, Square(3) returns 9 (discarded), Double(3) returns 6, so the delegate returns 6, which is printed.

❓ Predict Output

advanced

Effect of removing a method from a multicast delegate

What is the output of this C# program after removing a method from a multicast delegate?

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    static void Main()
    {
        Notify notify = MethodA;
        notify += MethodB;
        notify -= MethodA;
        notify();
    }

    static void MethodA() => Console.WriteLine("MethodA called");
    static void MethodB() => Console.WriteLine("MethodB called");
}

A MethodB called

B MethodA called\nMethodB called

C MethodA called

D No output

💡 Hint

Removing a method unsubscribes it from the delegate invocation list.

After removing MethodA, only MethodB remains in the delegate. So only MethodB is called, producing "MethodB called".

❓ Predict Output

advanced

Exception behavior in multicast delegates

What happens when a multicast delegate invokes multiple methods and one throws an exception?

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    static void Main()
    {
        Notify notify = MethodA;
        notify += MethodB;
        try
        {
            notify();
        }
        catch (Exception ex)
        {
            Console.WriteLine("Exception caught: " + ex.Message);
        }
    }

    static void MethodA() => Console.WriteLine("MethodA called");
    static void MethodB() => throw new InvalidOperationException("Error in MethodB");
}

A MethodA called\nMethodB called

B MethodA called\nException caught: Error in MethodB

C Exception caught: Error in MethodB

D No output

💡 Hint

Multicast delegates invoke methods in order until an exception occurs.

MethodA runs and prints its message. Then MethodB throws an exception, which stops further invocation. The exception is caught and its message printed.

🧠 Conceptual

expert

Number of methods in a multicast delegate

Given the following code, how many methods are in the multicast delegate 'notify' before invocation?

C Sharp (C#)

using System;

public delegate void Notify();

class Program
{
    static void Main()
    {
        Notify notify = MethodA;
        notify += MethodB;
        notify += MethodC;
        notify -= MethodB;
        notify += MethodD;
    }

    static void MethodA() { }
    static void MethodB() { }
    static void MethodC() { }
    static void MethodD() { }
}

A 1

B 4

C 2

D 3

💡 Hint

Count additions and removals carefully.

Initially, notify has MethodA (1). Then MethodB added (2), MethodC added (3). MethodB removed (back to 2). MethodD added (3). So total 3 methods before invocation.

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