OOP Best Practices - Writing Clean, Maintainable Object-Oriented Code in C#

Vaibhav • September 10, 2025

In the previous article, we explored object equality - how to compare instances correctly using reference and value semantics. Now, we step back and look at the bigger picture: how to apply object-oriented programming (OOP) principles effectively in real-world C# projects. This article focuses on OOP best practices - the habits, patterns, and design choices that help you write clean, maintainable, and scalable object-oriented code.

C# is a fully object-oriented language, and mastering its OOP capabilities is essential for building robust applications. Whether you're designing a small utility class or architecting a large system, these best practices will guide you in structuring your code around objects, responsibilities, and relationships.

Design Classes Around Responsibilities

Every class should represent a single, well-defined responsibility. This aligns with the Single Responsibility Principle (SRP), which states that a class should have one reason to change. When a class tries to do too much - like handling data, formatting output, and managing persistence - it becomes hard to understand and maintain.

public class Invoice
{
    public string Id { get; }
    public decimal Amount { get; }

    public Invoice(string id, decimal amount)
    {
        Id = id;
        Amount = amount;
    }

    public decimal CalculateTax() => Amount * 0.18m;
}

This class focuses only on representing an invoice and calculating tax. It doesn’t handle printing, saving, or emailing - those responsibilities belong elsewhere.

Encapsulate State and Behavior

Encapsulation is the cornerstone of OOP. It means hiding internal state and exposing behavior through controlled interfaces. This protects your objects from misuse and enforces consistency.

public class BankAccount
{
    private decimal balance;

    public void Deposit(decimal amount)
    {
        if (amount > 0)
            balance += amount;
    }

    public bool Withdraw(decimal amount)
    {
        if (amount > balance)
            return false;

        balance -= amount;
        return true;
    }

    public decimal GetBalance() => balance;
}

The balance field is private, and all interactions go through methods that enforce rules. This keeps the object in a valid state and makes its behavior predictable.

Use Properties Instead of Public Fields

Public fields expose internal implementation and allow uncontrolled access. Use properties to provide controlled access and maintain encapsulation. Properties can include validation, logging, or computed logic.

public class Product
{
    private decimal price;

    public decimal Price
    {
        get => price;
        set
        {
            if (value < 0)
                throw new ArgumentException("Price cannot be negative.");
            price = value;
        }
    }
}

This property enforces a rule and protects the internal field. External code cannot bypass the validation.

Favor Composition Over Inheritance

Inheritance creates tight coupling and fragile hierarchies. Composition - building classes by combining smaller components - is more flexible and easier to maintain. Use inheritance only when there is a clear “is-a” relationship and polymorphism is needed.

public class Engine
{
    public void Start() => Console.WriteLine("Engine started");
}

public class Car
{
    private Engine engine = new Engine();

    public void StartCar()
    {
        engine.Start();
        Console.WriteLine("Car is running");
    }
}

The Car class uses composition to delegate behavior to Engine. This keeps the design modular and testable.

Design for Extensibility

Your classes should be easy to extend without modifying existing code. This aligns with the Open/Closed Principle - classes should be open for extension but closed for modification. Use interfaces, abstract classes, and virtual methods to support extensibility.

public interface IPrinter
{
    void Print(string message);
}

public class ConsolePrinter : IPrinter
{
    public void Print(string message) => Console.WriteLine(message);
}

public class Logger
{
    private readonly IPrinter printer;

    public Logger(IPrinter printer)
    {
        this.printer = printer;
    }

    public void Log(string message)
    {
        printer.Print($"Log: {message}");
    }
}

The Logger class depends on an interface, not a concrete implementation. You can extend the system by adding new IPrinter implementations without changing Logger.

Use Constructors to Enforce Valid State

A class should be valid from the moment it is created. Use constructors to enforce required values and prevent incomplete objects. Avoid exposing setters for values that should not change after initialization.

public class User
{
    public string Username { get; }
    public DateTime RegisteredAt { get; }

    public User(string username)
    {
        if (string.IsNullOrWhiteSpace(username))
            throw new ArgumentException("Username is required.");

        Username = username;
        RegisteredAt = DateTime.Now;
    }
}

This constructor ensures that every User object starts in a valid state. The properties are read-only and cannot be changed later.

Avoid Hidden Side Effects

Methods should be predictable. Avoid hidden side effects like modifying global state or writing to disk unless clearly documented. Prefer pure methods - those that depend only on their inputs and produce consistent outputs.

public decimal CalculateTotal(decimal price, decimal taxRate)
{
    return price + (price * taxRate / 100);
}

This method is pure. It doesn’t modify any state or depend on external data. Pure methods are easier to test and reason about.

Use Meaningful Names

Naming is one of the hardest parts of programming. Choose names that clearly describe the purpose of the class, method, or property. Avoid vague names like DoWork() or Handle().

public class InvoiceGenerator
{
    public Invoice GenerateInvoice(Order order) { ... }
}

This name tells you exactly what the class and method do. It improves readability and discoverability.

Document Public APIs

Use XML comments to document the purpose of your classes, methods, and properties. This helps other developers understand how to use them correctly and improves IntelliSense support.

/// <summary>Represents a customer order.</summary>
public class Order
{
    /// <summary>Gets the total amount of the order.</summary>
    public decimal Total { get; }
}

These comments appear in tooltips and documentation tools. They make your code easier to use and maintain.

Design for Testability

A well-designed class is easy to test. Avoid static methods and tightly coupled dependencies. Use interfaces and dependency injection to make testing easier.

public class OrderProcessor
{
    private readonly IEmailSender emailSender;

    public OrderProcessor(IEmailSender sender)
    {
        emailSender = sender;
    }

    public void Process(Order order)
    {
        // Process order
        emailSender.SendConfirmation(order);
    }
}

This class depends on an interface, not a concrete implementation. You can pass a mock IEmailSender in tests to verify behavior.

Keep Classes Small and Focused

Large classes are hard to understand and maintain. Break them into smaller classes that each handle a specific responsibility. This improves readability and testability.

// Instead of one big Report class:
public class ReportGenerator { ... }
public class ReportFormatter { ... }
public class ReportPrinter { ... }

Each class handles one aspect of the report process. This separation of concerns makes the system easier to evolve.

Summary

Object-oriented programming is more than just using classes and objects - it’s about designing systems that are modular, maintainable, and expressive. In this article, we explored best practices for writing clean OOP code in C#: designing around responsibilities, encapsulating state, favoring composition, enforcing valid state, and documenting behavior.

We also discussed how to avoid hidden side effects, use meaningful names, and design for testability. These practices help you build systems that are easier to understand, easier to change, and more reliable in production.

As you continue your journey in C#, keep these principles in mind. They will guide you in writing better code, collaborating more effectively, and building software that lasts.

In the next article, we’ll begin Chapter 10 with Introduction to Inheritance - exploring how classes can extend behavior and support polymorphism.