Virtual method dispatch mechanism in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a virtual method named Speak.

C Sharp (C#)

public class Animal {
    public [1] void Speak() {
        Console.WriteLine("Animal speaks");
    }
}

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

Astatic

Boverride

Csealed

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'override' instead of 'virtual' in base class method declaration.

The virtual keyword allows a method to be overridden in derived classes, enabling virtual method dispatch.

2fill in blank

medium

Complete the code to override the Speak method in the Dog class.

C Sharp (C#)

public class Dog : Animal {
    public [1] void Speak() {
        Console.WriteLine("Dog barks");
    }
}

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

Avirtual

Bnew

Coverride

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' instead of 'override' in derived class method.

The override keyword is used to provide a new implementation of a virtual method in a derived class.

3fill in blank

hard

Fix the error in the method declaration to correctly override the base class method.

C Sharp (C#)

public class Cat : Animal {
    public [1] void Speak() {
        Console.WriteLine("Cat meows");
    }
}

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

Anew

Boverride

Cvirtual

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' or 'new' instead of 'override' in derived class.

To override a virtual method from the base class, the derived class method must use the override keyword.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps animal names to their Speak method outputs if the name length is greater than 3.

C Sharp (C#)

var sounds = new Dictionary<string, string> {
    {"Dog", "Bark"},
    {"Cat", "Meow"},
    {"Horse", "Neigh"}
};

var filtered = sounds.Where(kv => kv.Key.[1] > 3)
                      .ToDictionary(kv => kv.Key, kv => kv.[2]);

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

ALength

BValue

CKey

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Count' instead of 'Length' for string length.

Using 'Key' instead of 'Value' for dictionary value.

The Length property gets the length of the string key, and Value accesses the dictionary value.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase animal names to their sounds if the sound length is greater than 3.

C Sharp (C#)

var sounds = new Dictionary<string, string> {
    {"Dog", "Bark"},
    {"Cat", "Meow"},
    {"Horse", "Neigh"}
};

var result = sounds.Where(kv => kv.Value.[1] > 3)
                   .ToDictionary(kv => kv.Key.[2](), kv => kv.[3]);

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

ALength

BValue

CToUpper

DKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Count' instead of 'Length'.

Using 'Key' instead of 'Value' for the dictionary value.

The Length property checks the length of the sound string, ToUpper() converts the key to uppercase, and Value accesses the sound.

Practice

1. What does the virtual method dispatch mechanism in C# primarily allow?
virtual methods let child classes provide their own version of a method. What is the main benefit?

easy

A. It forces the program to call the base class method only.

B. It makes all methods static by default.

C. It disables method overriding in child classes.

D. It allows the program to decide at runtime which method version to call.

Solution

1. Step 1: Understand virtual method purpose

   Virtual methods allow child classes to override a method and provide their own implementation.

2. Step 2: Identify when method is chosen

   The actual method called is decided at runtime, based on the object's real type, not the variable's type.

3. Final Answer:

   It allows the program to decide at runtime which method version to call. -> Option D

4. Quick Check:

   Virtual method dispatch = runtime method choice [OK]

Hint: Virtual means runtime method choice, not compile-time [OK]

Common Mistakes:

• Thinking method is fixed at compile time
• Confusing virtual with static methods
• Assuming base method always runs

2. Which of the following is the correct syntax to declare a virtual method in a C# class?

easy

A. virtual public void Display() { }

B. public void virtual Display() { }

C. public virtual void Display() { }

D. public override void Display() { }

Solution

1. Step 1: Recall virtual method syntax

   The keyword virtual comes after the access modifier and before the return type and method name.

2. Step 2: Check each option

   public virtual void Display() { } matches correct syntax: public virtual void Display() { }. Options B and C have wrong order, D uses override which is for overriding, not declaring virtual.

3. Final Answer:

   public virtual void Display() { } -> Option C

4. Quick Check:

   virtual keyword after access modifier [OK]

Hint: virtual keyword goes right after access modifier [OK]

Common Mistakes:

• Placing virtual after method name
• Using override instead of virtual to declare
• Wrong keyword order

3. Consider the following code:

class Base {
    public virtual string GetName() => "Base";
}
class Derived : Base {
    public override string GetName() => "Derived";
}

Base obj = new Derived();
Console.WriteLine(obj.GetName());

What will be the output?

medium

A. Base

B. Derived

C. Compile-time error

D. Runtime exception

Solution

1. Step 1: Identify method overriding

   The Derived class overrides the virtual method GetName from Base.

2. Step 2: Understand virtual dispatch

   The variable obj is of type Base but holds a Derived object. Virtual dispatch calls the Derived version at runtime.

3. Final Answer:

   Derived -> Option B

4. Quick Check:

   Virtual method calls child's override [OK]

Hint: Virtual calls run child's method if overridden [OK]

Common Mistakes:

• Assuming base method runs due to variable type
• Confusing compile-time and runtime binding
• Expecting errors from override

4. What is wrong with this code snippet?

class Animal {
    public virtual void Speak() {
        Console.WriteLine("Animal speaks");
    }
}
class Dog : Animal {
    public void Speak() {
        Console.WriteLine("Dog barks");
    }
}

Animal a = new Dog();
a.Speak();

medium

A. Dog's Speak method should be marked override to override base virtual method.

B. Animal's Speak method should not be virtual.

C. Dog's Speak method should be static.

D. No error; code runs and prints "Dog barks".

Solution

1. Step 1: Check method overriding rules

   To override a virtual method, the child method must use override keyword.

2. Step 2: Analyze given code

   Dog's Speak method lacks override, so it hides base method instead of overriding. Virtual dispatch calls base method.

3. Final Answer:

   Dog's Speak method should be marked override to override base virtual method. -> Option A

4. Quick Check:

   Override keyword needed to override virtual method [OK]

Hint: Override keyword required to override virtual method [OK]

Common Mistakes:

• Forgetting override keyword in child method
• Assuming method hides base automatically
• Confusing virtual and override keywords

5. You have a base class Shape with a virtual method Draw(). Two derived classes Circle and Square override Draw(). You want to write a method that takes a list of Shape objects and calls Draw() on each, ensuring the correct derived method runs.

Which approach correctly uses virtual method dispatch to achieve this?

hard

A. Declare Draw() as virtual in Shape, override in derived classes, then call Draw() on each Shape reference in the list.

B. Declare Draw() as static in Shape and call it directly on the class.

C. Do not use virtual; instead, use type checking and cast each object to call the correct method.

D. Override Draw() in derived classes but call Shape.Draw() explicitly for all objects.

Solution

1. Step 1: Understand virtual method usage

   Declaring Draw() as virtual in base allows derived classes to override it.

2. Step 2: Use polymorphism in list iteration

   Calling Draw() on each Shape reference triggers virtual dispatch, running the correct derived method.

3. Final Answer:

   Declare Draw() as virtual in Shape, override in derived, call Draw() on each Shape reference. -> Option A

4. Quick Check:

   Virtual + override + call on base type = correct method run [OK]

Hint: Use virtual + override, call on base type for correct method [OK]

Common Mistakes:

• Using static methods which don't support polymorphism
• Manually casting instead of relying on virtual dispatch
• Calling base method explicitly ignoring overrides