Runtime polymorphism execution in C Sharp (C#) - Deep Dive

Overview - Runtime polymorphism execution

What is it?

Runtime polymorphism in C# means that the program decides which method to call while it is running, not when it is compiled. It allows objects of different classes to be treated as objects of a common base class, but each can behave differently when a method is called. This happens through method overriding and virtual methods. It helps write flexible and reusable code.

Why it matters

Without runtime polymorphism, programs would be rigid and repetitive because each object type would need separate handling. It solves the problem of changing behavior dynamically based on the actual object type, making code easier to extend and maintain. Imagine having to write separate code for every type of animal sound instead of just calling a common method that knows how to behave differently for each animal.

Where it fits

Before learning runtime polymorphism, you should understand classes, inheritance, and method overriding in C#. After this, you can explore design patterns like Strategy or Factory that use polymorphism to build scalable applications.

Mental Model

Core Idea

Runtime polymorphism lets a program decide which method to run based on the actual object type during execution, enabling flexible behavior through a common interface.

Think of it like...

It's like a universal remote control that can operate different devices. When you press a button, the remote sends a signal, but the device receiving it decides how to respond based on what it is—a TV, a DVD player, or a sound system.

BaseClass
  │
  ├─ DerivedClassA (overrides method)
  └─ DerivedClassB (overrides method)

At runtime:
  BaseClass reference → points to DerivedClassA object → calls DerivedClassA's method
  BaseClass reference → points to DerivedClassB object → calls DerivedClassB's method

Build-Up - 6 Steps

1

Foundation

Understanding classes and inheritance

Concept: Learn how classes can inherit properties and methods from other classes.

In C#, a class can inherit from another class using the ':' symbol. The child class gets all the features of the parent class and can add more or change them.

Result

You can create a new class that reuses code from an existing class.

Knowing inheritance is essential because runtime polymorphism depends on having a base class and derived classes.

2

Foundation

Method overriding basics

Concept: Learn how a derived class can replace a method from its base class.

In C#, if a base class method is marked as 'virtual', a derived class can use 'override' to provide its own version. This means the derived class changes how that method works.

Result

Calling the method on a derived class object uses the new version, not the base one.

Overriding allows different classes to have their own behavior for the same method name.

3

Intermediate

Virtual methods and dynamic dispatch

🤔Before reading on: Do you think the method called depends on the variable type or the object type? Commit to your answer.

Concept: Virtual methods enable the program to decide at runtime which method version to call based on the actual object type.

When a method is virtual, C# uses a mechanism called dynamic dispatch. If you have a base class reference pointing to a derived class object, calling the virtual method runs the derived class's version, not the base's.

Result

The program behaves differently depending on the real object, even if the reference type is the base class.

Understanding dynamic dispatch is key to grasping how runtime polymorphism works under the hood.

4

Intermediate

Using base class references for polymorphism

🤔Before reading on: Can a base class variable hold a derived class object? Yes or no? Commit to your answer.

Concept: A base class reference can point to any derived class object, allowing polymorphic behavior.

Example: BaseClass obj = new DerivedClass(); obj.VirtualMethod(); Here, even though obj is typed as BaseClass, the DerivedClass's method runs.

Result

You can write code that works with the base class but behaves differently for each derived class object.

This ability to treat different objects uniformly while preserving their unique behavior is the heart of polymorphism.

5

Advanced

Sealed and non-virtual methods impact

🤔Before reading on: What happens if a method is not virtual or is sealed? Does runtime polymorphism still apply? Commit to your answer.

Concept: Only virtual methods can be overridden and participate in runtime polymorphism; sealed methods prevent further overriding.

If a method is not marked virtual, the derived class cannot override it for polymorphism. If a virtual method is sealed in a derived class, no further overrides are allowed. Calling such methods uses compile-time binding.

Result

Methods without virtual or sealed keywords do not support runtime polymorphism, limiting flexibility.

Knowing these keywords helps control where polymorphism applies and prevents unintended behavior.

6

Expert

Runtime polymorphism performance and internals

🤔Before reading on: Do you think runtime polymorphism is slower than direct calls? Why or why not? Commit to your answer.

Concept: Runtime polymorphism uses a virtual method table (vtable) to find the correct method at runtime, which adds a small overhead compared to direct calls.

Each class with virtual methods has a vtable, a list of pointers to method implementations. When calling a virtual method, the program looks up the method in the vtable of the actual object type. This lookup is fast but slightly slower than direct calls.

Result

You get flexible behavior with minimal performance cost, but understanding this helps optimize critical code.

Knowing the vtable mechanism explains why polymorphism is powerful yet efficient, and when to avoid it for performance-critical paths.

Under the Hood

At runtime, each class with virtual methods has a virtual method table (vtable) that stores pointers to the actual method implementations. When a virtual method is called on a base class reference, the runtime uses the object's vtable to find and execute the correct overridden method. This process is called dynamic dispatch and happens behind the scenes, enabling the program to select the right behavior based on the object's real type.

Why designed this way?

This design allows flexibility and extensibility without changing existing code. Early languages used static binding, which was faster but inflexible. The vtable approach balances performance and dynamic behavior, making object-oriented programming practical and scalable. Alternatives like message passing were slower or more complex, so vtables became the standard.

Object Reference
    │
    ▼
+-----------------+
| Object Instance |
|  (DerivedClass) |
+-----------------+
        │
        ▼
+-----------------+
|   VTable Pointer |
+-----------------+
        │
        ▼
+-----------------------------+
| VTable (method pointers)    |
|  - Method1 → DerivedClass.M1|
|  - Method2 → DerivedClass.M2|
+-----------------------------+
        │
        ▼
+-----------------------------+
| Actual Method Implementation|
+-----------------------------+

Myth Busters - 4 Common Misconceptions

Quick: Does declaring a method 'virtual' mean it will always be overridden? Commit yes or no.

Common Belief: If a method is virtual, it must be overridden in derived classes.

Reality: Virtual methods can be overridden but don't have to be. If not overridden, the base class version runs.

Why it matters: Assuming all virtual methods are overridden can lead to unexpected behavior when base methods run instead.

Quick: Does runtime polymorphism work if the method is not marked virtual? Commit yes or no.

Common Belief: Any method can be overridden and called polymorphically regardless of virtual keyword.

Reality: Only methods marked virtual (or abstract/interface) support runtime polymorphism. Non-virtual methods use static binding.

Why it matters: Missing the virtual keyword means polymorphism won't work, causing bugs where the base method runs unexpectedly.

Quick: If a base class reference points to a derived object, does calling a non-virtual method run the derived version? Commit yes or no.

Common Belief: Base class references always call the derived class method if it exists.

Reality: Non-virtual methods are bound at compile time to the reference type, so the base class method runs even if the object is derived.

Why it matters: This misunderstanding causes confusion when expected overridden behavior does not happen.

Quick: Is runtime polymorphism free of performance cost? Commit yes or no.

Common Belief: Runtime polymorphism has no performance impact compared to direct calls.

Reality: It adds a small overhead due to vtable lookup, which is usually negligible but matters in tight loops or performance-critical code.

Why it matters: Ignoring this can lead to inefficient code in high-performance scenarios.

Expert Zone

1

Overriding a method with different accessibility (like making it less accessible) can cause subtle bugs or compiler errors.

2

The 'new' keyword in C# hides base methods but does not override them, which can confuse polymorphism behavior.

3

Interface methods are always virtual, but their dispatch mechanism differs slightly from class virtual methods.

When NOT to use

Avoid runtime polymorphism when performance is critical and method calls happen millions of times per second; consider using static polymorphism with generics or direct calls. Also, if behavior does not need to change dynamically, simpler designs without virtual methods are better.

Production Patterns

In real-world C# applications, runtime polymorphism is used in frameworks like ASP.NET for controllers, in dependency injection to swap implementations, and in UI event handling to respond differently to user actions. It enables writing extensible libraries where users can provide custom behavior by subclassing.

Connections

Dynamic dispatch in functional programming

Builds-on similar runtime decision making for function calls

Understanding runtime polymorphism helps grasp how some functional languages choose function implementations dynamically, bridging object-oriented and functional paradigms.

Human decision making under uncertainty

Shares the pattern of choosing actions based on context at runtime

Just like runtime polymorphism chooses behavior based on object type, humans adapt decisions based on current situations, showing a natural parallel between programming and cognition.

Network protocol message routing

Similar to dispatching messages to handlers based on message type

Knowing runtime polymorphism clarifies how network systems route different message types to appropriate handlers dynamically, improving system flexibility.

Common Pitfalls

#1 Calling a non-virtual method expecting polymorphic behavior

Wrong approach:

BaseClass obj = new DerivedClass();
obj.NonVirtualMethod(); // expects DerivedClass version but runs BaseClass

Correct approach:

BaseClass obj = new DerivedClass();
obj.VirtualMethod(); // virtual method overridden in DerivedClass

Root cause:

Not marking the method as virtual prevents runtime polymorphism, causing base method to run.

#2 Using 'new' keyword instead of 'override' for method overriding

Wrong approach:

public class DerivedClass : BaseClass {
public new void VirtualMethod() { /*...*/ }
}

Correct approach:

public class DerivedClass : BaseClass {
public override void VirtualMethod() { /*...*/ }
}

Root cause:

The 'new' keyword hides the base method but does not override it, breaking polymorphism.

#3 Sealing a method unintentionally stopping overrides

Wrong approach:

public sealed override void VirtualMethod() { /*...*/ } // no further overrides allowed

Correct approach:

public override void VirtualMethod() { /*...*/ } // allows further overrides

Root cause:

Using 'sealed' keyword on an override prevents further polymorphic behavior, which may be unintended.

Key Takeaways

Runtime polymorphism lets programs decide which method to run based on the actual object type during execution, enabling flexible and reusable code.

It requires virtual methods in the base class and overriding methods in derived classes to work correctly.

Base class references can point to derived objects, allowing uniform code to behave differently depending on the real object.

Behind the scenes, a virtual method table (vtable) enables fast method lookup at runtime with minimal performance cost.

Misunderstanding virtual, override, and new keywords leads to common bugs where polymorphism does not work as expected.