Introduction to SOLID Principles: A Beginner's Guide to Writing Better Code

When we start learning programming, most of our attention goes toward making things work. We learn how to create variables, write conditions, use loops, and build classes. Then we combine everything to create applications.

At this stage, our main goal is usually simple: write code that works and produces the expected result.

And there is nothing wrong with that. Making something work is an important part of learning programming.

But as we start building larger applications, we slowly realize that writing working code is only one part of software development.

Imagine that you have built a small online shopping application. Customers can browse products, add items to their cart, and place orders.

Everything works perfectly.

A few weeks later, you decide to add online payments. Then you want to introduce discount coupons, send order confirmation emails, and support different delivery methods.

These features sound simple enough. However, while adding them, you might notice a few problems.

A small change in one part of your application unexpectedly breaks something else. Some classes have become so large that understanding them takes a lot of time. You might even find yourself copying the same code into different places.

The application still works, but making changes is becoming increasingly difficult.

This is where software design becomes important.

Good software design helps us organize code so that it is easier to understand, modify, test, and maintain.

One of the most widely taught sets of guidelines for improving object-oriented software design is known as the SOLID principles.

In this article, we'll understand what SOLID principles are, why they matter, and how they can help us write better code.

You don't need any previous knowledge of SOLID or design patterns to follow this article.

What are software design principles?

Before learning SOLID, we need to understand what software design principles actually mean.

When we develop an application, there are usually many ways to solve the same problem.

For example, suppose we're building an application that manages customer orders.

We could write all the code for calculating prices, saving orders, processing payments, and sending emails inside a single class.

Alternatively, we could divide these responsibilities among different classes.

Both approaches might produce the same output.

So, how do we decide which approach is better?

This is where software design principles help us.

Software design principles are guidelines that help developers make better decisions about how to organize and structure their code.

They encourage us to think beyond whether our code works.

We also need to consider how easy it will be to understand the code, fix problems, introduce new features, and make changes in the future.

For example, a well-designed application should ideally allow us to modify the email notification system without having to change the code responsible for calculating order totals.

Design principles help us work toward that kind of separation.

However, it is important to understand that these principles are guidelines, not strict rules that must be followed in every situation.

Sometimes a simple solution is better than a complicated design that tries to follow every principle.

The goal is to make our code easier to work with, not to make it unnecessarily complicated.

What are SOLID principles?

SOLID is an acronym that represents five software design principles.

These principles are mainly associated with object-oriented programming (OOP), where we organize applications using classes and objects.

The five principles were brought together and popularized by software engineer Robert C. Martin, also known as Uncle Bob. The acronym SOLID was later introduced by Michael Feathers.

The name SOLID comes from the first letter of each principle.

At first, these names might sound complicated, especially if you're new to object-oriented programming.

Don't worry about memorizing them immediately.

The ideas behind them become much easier to understand when we look at simple examples.

Each principle addresses a different problem that developers may encounter while designing software.

Together, they can help us build applications that are easier to change and maintain.

Why do we need SOLID principles?

Let's return to our online shopping application.

When we first developed the application, we created a class called OrderManager.

The class was responsible for almost everything related to an order.

It calculated the total price, applied discounts, saved the order to the database, processed payments, and sent confirmation emails.

Initially, this seemed convenient. All the order-related operations were available in one place.

But as the application grew, this design started creating problems.

Suppose we wanted to change the email service used for sending confirmation messages.

We would have to modify the OrderManager class.

Now imagine that we also wanted to introduce a new payment method. Once again, we would have to modify the same class.

Even a change to how discounts are calculated could require modifying it.

Over time, the class becomes larger and more difficult to understand.

The problem is that too many unrelated responsibilities have become connected to the same piece of code.

This is an example of poor separation of responsibilities.

If we organize the application differently, we can make changes more safely.

For instance, payment processing can be handled by a payment-related component, while notifications can be handled separately.

When we change how emails are sent, we shouldn't need to change the code that calculates order totals.

This is the kind of problem SOLID principles help us identify and address.

Let's briefly explore each principle.

1. Single Responsibility Principle (SRP)

The Single Responsibility Principle is the first principle in SOLID.

It states that:

A class should have only one reason to change.

What does that mean?

Imagine that we have a class responsible for generating invoices.

The class calculates the invoice total, saves the invoice to a database, and sends the invoice to the customer through email.

Although all these operations are related to invoices, they are different responsibilities.

The rules for calculating an invoice might change because of new tax requirements.

The database code might change because we decide to use a different database system.

The email functionality might change because we switch to another email provider.

These are three different reasons for the same class to change.

Instead, we can separate these responsibilities into different classes or components.

One can handle invoice calculations, another can save invoices, and another can send emails.

Now, when the email system changes, we can focus on the email-related code without unnecessarily affecting invoice calculations.

This is the basic idea behind the Single Responsibility Principle.

It doesn't mean that every class must contain only one method. A class can have several methods as long as they work together toward the same responsibility.

We'll explore this principle with practical C# examples in a separate article.

2. Open/Closed Principle (OCP)

The second principle is the Open/Closed Principle.

It states that:

Software entities should be open for extension but closed for modification.

This statement may seem confusing at first.

How can something be open and closed at the same time?

Let's understand it with an example.

Imagine that our shopping application supports only one payment method: credit cards.

Later, we want to introduce UPI payments. A few months after that, we also want to support digital wallets.

One approach is to keep modifying the same payment-processing code whenever a new payment method is introduced.

As the number of payment methods increases, the code might become filled with conditions that check which payment method the customer selected.

Another approach is to design the payment system so that new payment methods can be added through separate implementations.

For example, we could introduce a common payment interface and create different implementations for credit card, UPI, and wallet payments.

The existing checkout code could work with the common interface without needing to know the details of every payment method.

This means we can extend the application's functionality without repeatedly changing the existing checkout logic.

That is the basic idea behind the Open/Closed Principle.

It doesn't mean that existing code should never be modified. We still need to fix bugs, improve code, and make necessary changes.

The principle encourages us to design parts of our application so that common future extensions don't require unnecessary modifications to stable, working code.

3. Liskov Substitution Principle (LSP)

The third principle is the Liskov Substitution Principle.

It is named after computer scientist Barbara Liskov.

The principle is about how different types of objects behave when we use inheritance or other forms of substitutability.

In simple terms:

If one type can be used in place of another, it should continue to behave in a way that the application expects.

Let's take an example.

Suppose our shopping application has a common payment interface with a method to process payments.

We create different classes to handle credit card payments and UPI payments.

Both classes follow the same interface.

Now imagine that our checkout system accepts any payment processor that follows this interface.

Whether we provide the credit card payment processor or the UPI payment processor, the checkout system should be able to use it according to the same basic expectations.

For example, both implementations should correctly follow the agreed rules for processing a payment and reporting its result.

If one implementation unexpectedly throws an unsupported-operation error whenever the checkout system tries to process a payment, even though the interface promises that capability, it may violate those expectations.

Simply sharing the same methods is not enough. The implementations must also follow the expected behavior.

This is what the Liskov Substitution Principle is concerned with.

Among the five SOLID principles, LSP can take a little more time to understand because it deals with behavior, not just the structure of our classes.

We'll explore it more carefully with examples in its dedicated article.

4. Interface Segregation Principle (ISP)

The fourth principle is the Interface Segregation Principle.

It states that:

Clients should not be forced to depend on methods they do not use.

To understand this, let's first talk about interfaces.

In object-oriented programming, an interface can describe a set of operations that a class agrees to provide.

For example, we might have an interface that describes how notifications are sent.

Now imagine that we create a large interface containing methods for sending emails, SMS messages, and push notifications.

What happens if we want to create a class that only sends emails?

Depending on the programming language and design, that class may be required to provide implementations for all the methods, including SMS and push notifications, even though it doesn't support them.

This is unnecessary.

Instead, we could create smaller interfaces for different notification capabilities.

An email notification class could implement the email-related interface, while an SMS notification class could implement the SMS-related interface.

This way, classes only need to depend on the operations that are relevant to them.

That is the idea behind Interface Segregation.

However, this doesn't mean we should create a separate interface for every single method.

The purpose is to keep interfaces focused and useful for the classes and components that depend on them.

5. Dependency Inversion Principle (DIP)

The fifth and final principle is the Dependency Inversion Principle.

It states that:

High-level and low-level modules should depend on abstractions, not directly on each other.

This is probably the most technical-sounding principle in SOLID.

Let's simplify it.

Imagine that our shopping application sends an email whenever a customer places an order.

We have an OrderService class that handles orders and an EmailSender class that sends emails.

If OrderService directly creates and uses a particular EmailSender implementation, the two classes become tightly connected.

What happens if we decide to switch to a different email provider?

We might need to modify OrderService, even though the order-processing rules haven't changed.

Now imagine that we introduce an interface called INotificationSender.

Instead of depending directly on a particular email-sending class, OrderService depends on this interface.

Different classes can provide different implementations of the interface.

For example, one implementation might send emails, while another might send SMS messages.

As long as they follow the agreed interface, OrderService can work with them without knowing their internal details.

This reduces the direct dependency between order processing and notification delivery.

In this example, OrderService is the higher-level component because it handles the application's order-related operations. The email-sending class is a lower-level component because it handles the technical details of sending messages.

By making both depend on a common abstraction, we can keep them more independent.

This is the idea behind the Dependency Inversion Principle.

You might have heard about Dependency Injection while learning C# or ASP.NET Core. It is a technique that can help us implement designs that follow the Dependency Inversion Principle.

However, Dependency Injection and Dependency Inversion are not the same thing. We'll understand that difference in the dedicated article.

Are SOLID principles design patterns?

A common misunderstanding among beginners is that SOLID principles are design patterns.

They are not.

SOLID principles are software design guidelines, while design patterns are reusable approaches to solving common software design problems.

For example, SOLID principles help us understand why we should avoid tightly connecting unrelated parts of an application.

A design pattern, such as the Strategy pattern, provides a particular way of organizing code to handle interchangeable behaviors.

In other words, principles guide our design decisions, while patterns give us established ways to address specific problems.

You may also notice that certain design patterns can help us follow one or more SOLID principles.

For example, the Strategy pattern can help us design functionality that can be extended without repeatedly modifying existing code.

But we don't need to learn every design pattern before understanding SOLID.

In fact, learning SOLID can make many design patterns easier to understand later.

Are SOLID principles only for C# and Java?

No.

SOLID principles are not specific to a programming language.

They are most closely associated with object-oriented software design and can be applied in languages such as C#, Java, Python, C++, and TypeScript.

The syntax and available language features may differ, but the underlying ideas remain similar.

For example, separating responsibilities is useful whether we're writing a C# application or a Python application.

Similarly, reducing unnecessary dependencies can help us organize code in many different environments.

In this tutorial series, we'll primarily use C# to demonstrate the principles because it supports object-oriented programming features such as classes, interfaces, and inheritance.

However, the concepts you learn can also be applied when working with other languages.

Do we need to follow all five principles in every application?

No, and this is something every beginner should understand.

SOLID principles are useful, but trying to apply all five principles everywhere can make an application unnecessarily complicated.

Imagine that you're writing a small program that adds two numbers and displays the result.

You don't need several classes, multiple interfaces, and dependency injection just to perform that calculation.

A simple solution is perfectly fine.

Similarly, not every class requires an interface, and not every piece of code needs to be designed for future extension.

Sometimes developers create too many layers of abstraction because they assume that more classes and interfaces automatically mean better software design.

That isn't necessarily true.

Good software design is about managing complexity, not adding complexity.

As developers, we need to understand the problem we're solving and make appropriate design decisions.

When an application grows and its requirements change, SOLID principles can help us identify areas where better separation and abstraction would make the code easier to maintain.

The goal is not to force every principle into every class.

Instead, we should understand these principles well enough to recognize when they can help.

What are the benefits of following SOLID principles?

Now that we have briefly explored all five principles, let's look at what they can help us achieve.

1. Easier maintenance

Software rarely stays the same forever.

We fix bugs, introduce new features, improve performance, and update existing functionality.

When classes have clear responsibilities and unnecessary dependencies are reduced, it becomes easier to identify which part of the application needs to change.

2. Easier testing

Suppose we want to test how an application calculates the total amount for an order.

If the calculation code is tightly connected to a database, payment gateway, and email service, testing it may become difficult.

When responsibilities are separated and dependencies are designed carefully, we can often test the calculation logic without involving those unrelated systems.

This makes tests easier to write and manage.

3. Easier extension

Requirements change as applications grow.

A shopping application that initially supports credit cards might later need UPI payments or digital wallets.

A well-designed system can make it easier to introduce these features without repeatedly changing large sections of existing code.

4. Better readability

Imagine opening a class containing thousands of lines of code that handles many unrelated operations.

Understanding how everything works might take considerable effort.

Smaller, focused components with clear responsibilities can make it easier to understand what each part of the application does.

5. Reduced risk when making changes

When unrelated parts of an application are tightly connected, a small modification can unexpectedly affect other features.

Good separation cannot eliminate bugs, but it can help limit the number of components affected by a change.

This can make applications safer and easier to maintain over time.

How should beginners learn SOLID principles?

If you're completely new to software design, don't try to memorize all five principles at once.

Start by understanding the problems each principle is trying to solve.

For example, before learning the Single Responsibility Principle, try to understand why placing too many unrelated operations inside one class can become a problem.

Then look at how separating those responsibilities can improve the design.

The same approach works for the other principles.

First, understand the problem. Then study the principle and see how it helps solve that problem.

It's also useful to compare code that violates a principle with code that follows it.

This makes the difference easier to understand than simply memorizing definitions.

You don't need to apply every principle perfectly when you're starting out. As you build more applications and work with larger codebases, these ideas will gradually become more familiar.

What's next in this series?

This article is the introduction to our SOLID principles tutorial series.

In the upcoming articles, we'll explore each principle individually, starting with the Single Responsibility Principle.

For every principle, we'll begin with a simple explanation, look at a practical problem, and use C# examples to understand how the principle can improve our code.

We'll also discuss common mistakes and situations where applying a particular principle may not be necessary.

By the end of the series, you should have a better understanding of not just what the five SOLID principles mean, but why developers use them and how to recognize situations where they can be helpful.

Conclusion

When we're learning programming, it's natural to focus on getting our code to work.

But as our applications grow, we need to start thinking about how our code is organized.

A feature that takes only a few lines of code today might become much more complicated when new requirements are introduced.

SOLID principles give us a set of guidelines for dealing with some of these challenges.

They encourage us to separate responsibilities, design for extension, respect expected behavior, create focused interfaces, and reduce unnecessary dependencies.

You don't have to understand all five principles immediately. What matters is recognizing that the way we organize code can make a big difference as an application grows.

In the next article, we'll explore the Single Responsibility Principle (SRP) in detail and learn how separating responsibilities can make our C# code easier to understand and maintain.