Virtual method dispatch mechanism in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of virtual method call with base reference

What is the output of the following C# code?

C Sharp (C#)

using System;
class Animal {
    public virtual void Speak() {
        Console.WriteLine("Animal speaks");
    }
}
class Dog : Animal {
    public override void Speak() {
        Console.WriteLine("Dog barks");
    }
}
class Program {
    static void Main() {
        Animal a = new Dog();
        a.Speak();
    }
}

A Dog barks

B Runtime exception

C Compilation error

D Animal speaks

💡 Hint

Remember that virtual methods call the most derived override at runtime.

The variable 'a' is of type Animal but refers to a Dog object. Because Speak() is virtual and overridden in Dog, the Dog's Speak() method runs, printing 'Dog barks'.

❓ Predict Output

intermediate

Output when calling non-virtual method on derived class

What will this C# program print?

C Sharp (C#)

using System;
class Base {
    public void Show() {
        Console.WriteLine("Base Show");
    }
}
class Derived : Base {
    public new void Show() {
        Console.WriteLine("Derived Show");
    }
}
class Program {
    static void Main() {
        Base b = new Derived();
        b.Show();
    }
}

A Derived Show

B Base Show

C Compilation error

D Runtime exception

💡 Hint

Non-virtual methods are called based on the variable's declared type.

The Show() method is not virtual. The variable 'b' is of type Base, so Base.Show() is called, printing 'Base Show'.

🔧 Debug

advanced

Identify the runtime error in virtual method dispatch

What error will this C# code produce when run?

C Sharp (C#)

using System;
class Parent {
    public virtual void Display() {
        Console.WriteLine("Parent Display");
    }
}
class Child : Parent {
    public override void Display() {
        throw new NullReferenceException();
    }
}
class Program {
    static void Main() {
        Parent p = new Child();
        p.Display();
    }
}

A No output, program runs normally

B Compilation error due to override

C StackOverflowException at runtime

D NullReferenceException at runtime

💡 Hint

Look at what the overridden method does when called.

The Child's Display method throws a NullReferenceException explicitly. When p.Display() runs, this exception is thrown at runtime.

🧠 Conceptual

advanced

Effect of sealed keyword on virtual method dispatch

Consider this C# code snippet. What will be the output?

C Sharp (C#)

using System;
class A {
    public virtual void Print() {
        Console.WriteLine("A Print");
    }
}
class B : A {
    public sealed override void Print() {
        Console.WriteLine("B Print");
    }
}
class C : B {
    public override void Print() {
        Console.WriteLine("C Print");
    }
}
class Program {
    static void Main() {
        A obj = new C();
        obj.Print();
    }
}

A A Print

B B Print

C Compilation error

D C Print

💡 Hint

The sealed keyword prevents further overrides.

Class B seals the Print method, so class C cannot override it. This causes a compilation error.

❓ Predict Output

expert

Output of virtual method calls with multiple inheritance levels

What is the output of this C# program?

C Sharp (C#)

using System;
class X {
    public virtual void Foo() {
        Console.WriteLine("X Foo");
    }
}
class Y : X {
    public override void Foo() {
        Console.WriteLine("Y Foo");
    }
}
class Z : Y {
    public override void Foo() {
        Console.WriteLine("Z Foo");
    }
}
class Program {
    static void Main() {
        X obj = new Z();
        obj.Foo();
        Y yobj = new Z();
        yobj.Foo();
    }
}

A Z Foo\nZ Foo

B X Foo\nY Foo

C Y Foo\nZ Foo

D Compilation error

💡 Hint

Virtual methods call the most derived override regardless of reference type.

Both obj and yobj refer to Z instances. Foo() is overridden in Z, so both calls print 'Z Foo'.