Virtual method dispatch mechanism in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Virtual method dispatch mechanism

Create base class object

↓

Call virtual method

↓

Check actual object type

↓

If base type

→Call base method

↓

If derived type

→Call overridden method

↓

Return method result

When calling a virtual method, the program checks the actual object's type at runtime and calls the correct method version.

Execution Sample

C Sharp (C#)

class Animal {
  public virtual string Speak() => "Animal sound";
}
class Dog : Animal {
  public override string Speak() => "Bark";
}
Animal a = new Dog();
Console.WriteLine(a.Speak());

This code creates a Dog object but stores it as an Animal reference, then calls Speak(), which runs Dog's version.

Execution Table

Step	Action	Object Type	Method Called	Output
1	Create Dog object assigned to Animal reference	Dog	N/A	N/A
2	Call Speak() on Animal reference	Dog	Dog.Speak()	"Bark"
3	Print output	Dog	N/A	Bark
4	End of execution	N/A	N/A	N/A

💡 Execution stops after printing the overridden method output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
a	null	Dog object reference	Dog object reference	Dog object reference

Key Moments - 2 Insights

Why does calling Speak() on an Animal reference run Dog's Speak()?

Because the method is virtual, the program checks the actual object type (Dog) at runtime and calls the overridden method (Dog.Speak()), as shown in execution_table step 2.

What if Speak() was not virtual, which method would run?

If Speak() was not virtual, the method called would be based on the reference type (Animal), so Animal.Speak() would run, not Dog.Speak(). This is different from the virtual dispatch shown in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what method is called at step 2?

A Dog.Speak()

B Animal.Speak()

C Object.ToString()

D Base class constructor

💡 Hint

Check the 'Method Called' column in execution_table row for step 2.

At which step is the Dog object created and assigned?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for object creation.

If Speak() was not virtual, what would be the output at step 3?

A "Bark"

B Compilation error

C "Animal sound"

D No output

💡 Hint

Refer to key_moments explanation about non-virtual method calls.

Concept Snapshot

Virtual method dispatch means calling a method on a base class reference runs the overridden method in the actual object's class.
Use 'virtual' keyword in base class and 'override' in derived class.
At runtime, the program checks the real object type to decide which method to run.
This enables flexible and dynamic behavior in object-oriented programming.

Full Transcript

This example shows how virtual method dispatch works in C#. We create a Dog object but store it in an Animal variable. When we call Speak() on the Animal variable, the program checks the actual object type at runtime, which is Dog, and calls Dog's Speak() method. This is because Speak() is marked virtual in Animal and overridden in Dog. The execution table shows each step: creating the Dog object, calling Speak(), and printing the output. Key moments explain why the overridden method runs and what would happen if the method was not virtual. The visual quiz tests understanding of method calls and object creation steps. Virtual dispatch allows programs to decide which method to run based on the real object, not just the reference type.

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