Virtual method dispatch mechanism in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the virtual method dispatch mechanism in C#?

It is the process where the program decides at runtime which version of a virtual method to call, based on the actual object's type, not the reference type.

beginner

How do you declare a method as virtual in C#?

By using the virtual keyword before the method return type in the base class.

beginner

What keyword is used to override a virtual method in a derived class?

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

intermediate

Why is virtual method dispatch important in object-oriented programming?

It enables polymorphism, allowing objects of different classes to be treated uniformly while invoking their specific behaviors at runtime.

intermediate

What happens if a method is not declared virtual but is redefined in a derived class?

The method in the derived class hides the base class method, but calls are resolved at compile time based on the reference type, not the object type.

Which keyword in C# allows a method to be overridden in derived classes?

A virtual

B override

C new

D abstract

The virtual keyword marks a method in the base class as overridable.

What determines which method implementation is called when using virtual methods?

A The object type at runtime

B The reference type

C The method name only

D The compiler settings

Virtual method dispatch uses the actual object's type at runtime to decide which method to call.

If a base class method is not virtual, what happens when a derived class defines a method with the same name?

A The derived method overrides the base method

B The base method is called always

C A compile error occurs

D The derived method hides the base method

Without virtual, the derived method hides the base method, and calls depend on reference type.

Which keyword must be used in the derived class to provide a new implementation of a virtual method?

A virtual

B new

C override

D sealed

The override keyword tells the compiler this method replaces the base virtual method.

What is the main benefit of virtual method dispatch?

A Faster code execution

B Runtime polymorphism

C Compile-time method selection

D Memory optimization

Virtual method dispatch enables runtime polymorphism, allowing flexible and dynamic method calls.

Explain how virtual method dispatch works in C# with an example.

Think about a base class with a virtual method and a derived class overriding it.

Describe the difference between method overriding and method hiding in C#.

Consider what happens when the base method is virtual or not.

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