Class Design Principles - Building Robust and Maintainable Types in C#

Vaibhav • September 10, 2025

In the previous article, we explored the object lifecycle - how objects are created, used, and cleaned up in memory. We discussed constructors, destructors, and the role of the garbage collector. Now, we shift our focus to the art of designing classes themselves. This article introduces class design principles - the foundational guidelines that help you build classes that are clear, cohesive, and maintainable.

A class is more than just a container for fields and methods. It represents a concept, a responsibility, and a contract. Good class design makes your code easier to understand, test, and evolve. Poor design leads to confusion, duplication, and bugs. In this article, we’ll explore how to structure classes thoughtfully, how to apply object-oriented principles, and how to avoid common pitfalls.

Designing for a Single Responsibility

One of the most important principles in class design is the Single Responsibility Principle (SRP). A class should represent one concept and have one reason to change. This makes it easier 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()
    {
        return Amount * 0.18m;
    }
}

This class focuses on representing an invoice and calculating tax. It does not handle printing, saving to a database, or sending emails. Those responsibilities belong elsewhere.

Encapsulating State and Behavior

A class should encapsulate its internal state and expose behavior through methods and properties. This protects the object 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()
    {
        return balance;
    }
}

This class hides its internal balance field and provides methods to interact with it. The methods enforce rules and prevent invalid operations.

Favor Composition Over Inheritance

Inheritance allows one class to extend another, but it can lead to tight coupling and fragile hierarchies. Prefer composition - building classes by combining smaller components.

public class Address
{
    public string City { get; set; }
    public string Country { get; set; }
}

public class User
{
    public string Username { get; set; }
    public Address Location { get; set; }
}

Here, the User class contains an Address object. This is composition - the user “has an” address. It’s more flexible than inheritance and easier to evolve.

Designing Clear and Consistent APIs

A class’s public methods and properties form its API - the interface that other code interacts with. This API should be clear, consistent, and intuitive.

public class Timer
{
    public void Start() { ... }
    public void Stop() { ... }
    public TimeSpan Elapsed { get; }
}

The method names are verbs that describe actions. The property name is a noun that describes a value. This naming convention makes the class easy to use and understand.

Using 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.

public class Product
{
    public string Name { get; }
    public decimal Price { get; }

    public Product(string name, decimal price)
    {
        if (string.IsNullOrWhiteSpace(name))
            throw new ArgumentException("Name is required.");

        if (price < 0)
            throw new ArgumentException("Price cannot be negative.");

        Name = name;
        Price = price;
    }
}

This constructor validates input and ensures that every Product object starts in a valid state.

Avoiding Public Fields

Public fields expose internal implementation and allow uncontrolled access. Use properties instead to provide controlled access and maintain encapsulation.

// ❌ Avoid this
public string Name;

// ✅ Use this
public string Name { get; set; }

Properties allow you to add validation, logging, or computed logic later without changing the public interface.

Keeping 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.

Using Access Modifiers Thoughtfully

Access modifiers control visibility. Use private for internal details, public for the API, and protected for inheritance scenarios. Avoid exposing members unnecessarily.

private void Validate() { ... }
public void Submit() { ... }

This keeps the class’s internal logic hidden and exposes only what’s needed for external use.

Designing 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.

Avoiding 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 when possible.

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

This method is pure - it depends only on its inputs and produces a consistent output. Pure methods are easier to test and reason about.

Documenting Class Behavior

Use XML comments to document the purpose of the class, its methods, and its properties. This helps other developers understand how to use it correctly.

/// <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 IntelliSense and improve discoverability.

Summary

Class design is a foundational skill in object-oriented programming. A well-designed class is focused, encapsulated, and easy to use. It exposes meaningful behavior, protects internal state, and supports long-term maintainability.

In this article, we explored principles like single responsibility, encapsulation, composition, API clarity, constructor validation, and testability. We discussed how to use access modifiers, avoid public fields, and document behavior.

As you continue building applications in C#, apply these principles to every class you write. Thoughtful class design leads to better code, fewer bugs, and a more enjoyable development experience.

In the next article, we’ll explore Composition vs Inheritance - two fundamental techniques for structuring relationships between classes.