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.