5 Common C# Polymorphism Mistakes to Avoid

Polymorphism in C# simplifies code by letting objects share a common base class, but misuse can lead to bugs and confusion. Here are 5 common mistakes to avoid:

• Skipping virtual and override: Always use virtual in base classes and override in derived classes to ensure proper behavior.
• Overusing Casting: Avoid explicit type checks and rely on base class methods for shared behavior.
• Confusing Method Hiding and Overriding: Use override for polymorphism and avoid new unless method hiding is intentional.
• Poor Base Class Design: Keep base classes focused and avoid overly broad or deep hierarchies.
• Mismanaging Polymorphic Collections: Define shared behaviors in base classes to avoid type checks and casting.

Mastering these principles ensures cleaner, more maintainable code. Avoid these pitfalls to make the most of polymorphism in C#.

1. Neglecting Virtual and Override Keywords

One common mistake in C# polymorphism is misusing the virtual and override keywords. For derived classes to properly override base class methods, the base methods must be marked with the virtual keyword. Without it, polymorphism won't work as expected, leading to runtime surprises and tricky bugs.

Take a look at this example:

class Shape
{
    public void Draw() // No virtual keyword here
    {
        Console.WriteLine("Drawing a shape");
    }
}

class Circle : Shape
{
    public void Draw() // This hides the base method instead of overriding it
    {
        Console.WriteLine("Drawing a circle");
    }
}

In this scenario, calling Draw() on a Shape reference that points to a Circle object will execute the base class's Draw() method, not the one in Circle. To fix this, you need to use the virtual and override keywords:

class Shape
{
    public virtual void Draw()
    {
        Console.WriteLine("Drawing a shape");
    }
}

class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a circle");
    }
}

If the override keyword is omitted in the derived class, the method will hide the base method instead of overriding it. This creates a disconnect between the two methods, potentially causing unexpected behavior during runtime.

To avoid these problems:

• Always mark base class methods intended for polymorphism with virtual.
• Use the override keyword in derived classes to explicitly override base methods.
• Leverage code analyzers to detect missing or incorrect keyword usage.

Using virtual and override correctly ensures your polymorphic code behaves as intended. However, keep in mind that other mistakes, like improper casting, can also cause issues in your code.

2. Incorrect Use of Casting

When working with polymorphism, avoiding explicit casting is key to writing clean, maintainable code. Relying on casting can lead to runtime errors and make your code harder to manage.

Take a look at this example where explicit casting is used unnecessarily:

public class Menagerie
{
    private List<Animal> animals = new List<Animal>();

    public void MakeAnimalsSounds()
    {
        foreach(var animal in animals)
        {
            if(animal is Cat)
            {
                ((Cat)animal).Meow();
            }
            else if(animal is Dog)
            {
                ((Dog)animal).Bark();
            }
        }
    }
}

This code has some clear problems:

• Hard to Maintain: Adding a new animal type means you have to modify the existing logic, increasing the risk of bugs.
• Violates Polymorphism: Instead of focusing on shared behavior, it relies on specific types.

A better approach is to use polymorphism by defining a shared interface or base class:

public abstract class Animal
{
    public virtual void MakeSound() { }
}

public class Cat : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Meow!");
    }
}

public class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Woof!");
    }
}

With this setup, the Menagerie class becomes much cleaner:

public class Menagerie
{
    private List<Animal> animals = new List<Animal>();

    public void MakeAnimalsSounds()
    {
        foreach(var animal in animals)
        {
            animal.MakeSound();
        }
    }
}

To avoid casting problems:

• Design with Polymorphism in Mind: Use abstract classes or interfaces with virtual methods to define shared behavior.
• Trust the Base Class: Stick to base class references and avoid checking for specific types.

3. Ignoring Method Hiding

Method hiding in C# can create unexpected behavior, especially when developers mistake it for method overriding or misuse the new keyword. This can disrupt polymorphism, which is meant to ensure behavior is determined by the actual object type.

Here’s an example of method hiding that can cause confusion:

public class BaseComponent
{
    public void Initialize()
    {
        Console.WriteLine("Base initialization");
    }
}

public class DerivedComponent : BaseComponent
{
    public new void Initialize()
    {
        Console.WriteLine("Derived initialization");
    }
}

In this case, the output of Initialize() depends on the reference type, not the actual object type. This breaks polymorphism and can lead to bugs:

BaseComponent component = new DerivedComponent();
component.Initialize(); // Outputs: "Base initialization"

DerivedComponent derived = new DerivedComponent();
derived.Initialize(); // Outputs: "Derived initialization"

To fix this, rely on the virtual/override pattern for polymorphic behavior or clearly document the use of method hiding with the new keyword:

// Correct polymorphism
public class BaseComponent
{
    public virtual void Initialize()
    {
        Console.WriteLine("Base initialization");
    }
}

public class DerivedComponent : BaseComponent
{
    public override void Initialize()
    {
        Console.WriteLine("Derived initialization");
    }
}

To avoid issues with method hiding:

• Use override for polymorphism: Make base methods virtual and override them in derived classes.
• Pay attention to compiler warnings about method hiding, as they often signal potential problems.
• If method hiding is necessary, explicitly use the new keyword and document why it’s being used.

While method hiding has its place, misusing it - or base classes in general - can lead to challenges in maintaining polymorphic designs ^[2][3].

sbb-itb-29cd4f6

4. Improper Use of Base Classes

Poorly designed base classes can disrupt polymorphism throughout your application, making it harder to maintain and extend. Common pitfalls include creating overly broad hierarchies, misusing method modifiers, and relying too heavily on inheritance.

"Polymorphism lets you focus on the public API, not implementation details." ^[1]

Here’s an example of a well-structured implementation that supports polymorphic behavior effectively:

public abstract class Shape
{
    public abstract double GetArea();
}

public class Circle : Shape
{
    private double radius;
    public override double GetArea()
    {
        return Math.PI * radius * radius;
    }
}

This setup ensures that all derived classes implement the GetArea method, maintaining consistent behavior across different shapes.

Principles for Better Base Class Design

To create base classes that enhance polymorphism, keep these principles in mind:

Practical Tips

• Use abstract classes to define templates for shared behavior.
• Avoid deep inheritance hierarchies that complicate your codebase.
• Focus on single responsibilities to keep base classes clean and manageable.

Effective base class design not only supports polymorphism but also simplifies the management of polymorphic collections - another area where developers often encounter challenges.

5. Mishandling Polymorphic Collections

Polymorphic collections in C# let you store objects of various types that share a common base class. While useful, improper handling of these collections can lead to runtime errors like InvalidCastException and make your code harder to maintain.

Here’s an example of a problematic implementation:

List<Animal> animals = new List<Animal>();
animals.Add(new Cat());
animals.Add(new Dog());

foreach (Animal animal in animals)
{
    if (animal is Cat)
    {
        ((Cat)animal).Purr(); // Explicit casting - risky and error-prone
    }
    else if (animal is Dog)
    {
        ((Dog)animal).Bark();
    }
}

This approach relies on type checking and explicit casting, which can be fragile and introduces potential issues at runtime.

A Cleaner Approach

A better way to handle polymorphic collections is by designing base classes with abstract or virtual methods. This allows derived classes to implement specific behaviors without requiring type checks:

public abstract class Animal
{
    public abstract void MakeNoise();
}

public class Cat : Animal
{
    public override void MakeNoise()
    {
        // Cat-specific implementation
    }
}

// Usage
List<Animal> animals = new List<Animal>();
foreach (Animal animal in animals)
{
    animal.MakeNoise(); // Polymorphic behavior in action
}

This approach ensures that behavior is defined in the base class and overridden in derived classes, eliminating the need for casting or type checks.

Best Practices for Working with Polymorphic Collections

To get the most out of polymorphic collections, follow these guidelines:

• Use virtual or abstract methods in base classes to define shared behaviors.
• Implement focused interfaces to establish clear functionality.
• Work with interfaces or base classes instead of specific implementations.
• Avoid type checks and explicit casting, as they can lead to brittle code.

For instance, when managing a collection of UI components, define shared behaviors in the base class:

public abstract class UIComponent
{
    public abstract void Render();
    public abstract void Update();
}

Wrapping Up

Avoiding common mistakes in polymorphism is key to writing better C# code. By mastering this concept, you can create applications that are easier to manage and scale. The issues we've discussed - like skipping the use of virtual and override keywords or mishandling polymorphic collections - can have a big impact on how your code performs and evolves.

When used properly, polymorphism offers clear advantages:

• Less Repetitive Code: It reduces the need for duplicating logic across derived classes.
• Easier Maintenance: Well-structured polymorphic designs make it simpler to update or expand functionality without causing issues.
• Better Type Safety: It minimizes runtime errors, leading to more reliable code.

Polymorphism isn’t just about inheritance or overriding methods - it’s about designing systems that are adaptable and easier to work with. By steering clear of these common pitfalls, you’ll write code that stands the test of time.

Want to stay sharp on C# and .NET practices? Subscribe to the .NET Newsletter at dotnetnews.co for regular tips and updates.

Polymorphism is a skill you refine over time, but avoiding these errors is a great step toward writing cleaner, professional-grade code.

FAQs

Let’s clear up some common questions about polymorphism in C# to help you better understand how it works.

What is the difference between virtual and override in C#?

The virtual keyword in a base class allows its methods to be overridden in derived classes. On the other hand, the override keyword is used in the derived class to provide a new implementation for the base method. Mixing these up can lead to runtime errors, as mentioned in Section 1.

Here’s an example to make it clear:

class Shape {
    public virtual void Draw() { 
        Console.WriteLine("Drawing a shape"); 
    }
}

class Circle : Shape {
    public override void Draw() { 
        Console.WriteLine("Drawing a circle"); 
    }
}

Using virtual in the base class ensures that derived class methods are called based on the object’s actual type. Without it, you might accidentally hide methods, causing unexpected results at runtime ^[3].

How do casting and polymorphic collections affect polymorphism?

Improper casting or handling of polymorphic collections can break polymorphism. Relying on explicit type checks or casts can make your code fragile and error-prone. Instead, use virtual methods or interfaces to define shared behavior across your class hierarchy.

Related posts

• C# Property Injection: Implementation Guide
• Top 7 Dependency Injection Best Practices for .NET
• C# Method Overloading Examples - Beginner Guide
• SOLID Principles in C#: Practical Examples