Software Design · Beginner to Advanced

SOLID Principles: Complete Developer Learning Guide

Understand the five object-oriented design principles with practical C# and ASP.NET Core examples, refactoring techniques, testing guidance, and real-world design considerations.

Writing code that works is only one part of software development. As an application grows, new requirements, integrations, and business rules can make classes difficult to change and test. A design that is easy to understand today may become tightly coupled and fragile over time.

SOLID is a group of five object-oriented design principles that help developers organize responsibilities, manage dependencies, and make software easier to extend. The principles are generic and language-independent. In this guide, we explain them generally and use C# and ASP.NET Core examples to connect them to practical .NET development.

The goal is not to add interfaces or classes everywhere. The goal is to create a design in which each part has a clear purpose and changes can be made with minimal unintended impact.

Learning roadmap

1. What are SOLID principles?
2. Why SOLID matters
3. Single Responsibility Principle
4. Open/Closed Principle
5. Liskov Substitution Principle
6. Interface Segregation Principle
7. Dependency Inversion Principle
8. How the principles work together
9. Applying SOLID in ASP.NET Core
10. SOLID and testing
11. Misconceptions and overengineering
12. Practical refactoring walkthrough
13. Design review checklist
14. Practice roadmap
15. Interview questions
16. Official resources

1. What are SOLID principles?

SOLID is an acronym for five principles associated with object-oriented software design. The acronym was popularized by Robert C. Martin, while the underlying ideas have contributions and history from several software design authors.

Letter	Principle	Core idea
S	Single Responsibility Principle (SRP)	A module should have one reason to change.
O	Open/Closed Principle (OCP)	Software entities should be open for extension but closed for modification.
L	Liskov Substitution Principle (LSP)	Subtypes should be usable wherever their base types are expected without breaking correctness.
I	Interface Segregation Principle (ISP)	Clients should not be forced to depend on methods they do not use.
D	Dependency Inversion Principle (DIP)	High-level policy and low-level details should depend on abstractions.

These are design guidelines, not language features or compiler rules. C# does not automatically enforce SOLID. Developers apply the principles through class design, interfaces, composition, dependency injection, and tests.

Important: SOLID is not a mandatory architecture or a checklist that every class must satisfy mechanically. It is a set of design heuristics for evaluating and improving software structure.

2. Why SOLID matters

Consider an ASP.NET Core application that manages orders. One service validates an order, calculates totals, saves it to SQL Server, sends an email, and writes an audit record. It may work initially, but unrelated requirements now force changes to the same class.

• High coupling: A class knows too much about concrete implementations of other components.
• Low cohesion: Unrelated responsibilities are grouped together.
• Difficult testing: Testing one business rule requires setting up databases or external providers.
• Risky changes: A small modification can affect unrelated behavior.
• Repeated conditionals: New behavior requires editing many switch statements or large methods.
• Fragile inheritance: A derived type does not behave as callers expect from its base type.

SOLID encourages clearer responsibilities, deliberate extension points, and dependencies that can be replaced or tested independently.

Do not apply SOLID mechanically. A small, stable class does not need multiple interfaces and layers just to satisfy an acronym. Design should reduce real complexity, not create ceremony.

3. S — Single Responsibility Principle (SRP)

Definition

A class or module should have one reason to change. In practical terms, it should have one cohesive responsibility and should be affected by one closely related area of business or technical change.

SRP does not mean a class can contain only one method. Several methods can belong together when they support the same responsibility.

Example: too many responsibilities

An order service that calculates totals, saves data, and sends email has several independent reasons to change.

public class OrderService
{
    public decimal CalculateTotal(Order order)
    {
        return order.Items.Sum(
            item => item.Quantity * item.UnitPrice);
    }

    public void Save(Order order)
    {
        // Direct database persistence
    }

    public void SendConfirmation(Order order)
    {
        // Direct email implementation
    }
}

Refactor by responsibility

Separate business calculation, persistence, and notification. The application service coordinates the use case while collaborators own their implementation details.

public interface IOrderRepository
{
    Task AddAsync(Order order, CancellationToken ct);
}

public interface IOrderNotifier
{
    Task SendConfirmationAsync(Order order, CancellationToken ct);
}

public class OrderPricingService
{
    public decimal CalculateTotal(Order order)
        => order.Items.Sum(x => x.Quantity * x.UnitPrice);
}

public class OrderApplicationService
{
    private readonly IOrderRepository _repository;
    private readonly IOrderNotifier _notifier;
    private readonly OrderPricingService _pricing;

    public OrderApplicationService(
        IOrderRepository repository,
        IOrderNotifier notifier,
        OrderPricingService pricing)
    {
        _repository = repository;
        _notifier = notifier;
        _pricing = pricing;
    }

    public async Task PlaceAsync(Order order, CancellationToken ct)
    {
        order.Total = _pricing.CalculateTotal(order);
        await _repository.AddAsync(order, ct);
        await _notifier.SendConfirmationAsync(order, ct);
    }
}

The application service still coordinates several steps, but it does not implement the details of database access, email delivery, or pricing calculation.

SRP in ASP.NET Core

• Controller: Handles HTTP concerns such as route binding and response mapping.
• Application service: Coordinates a use case or workflow.
• Domain service: Holds domain logic that does not naturally belong to one entity.
• Repository: Encapsulates persistence operations when that abstraction is useful.
• Infrastructure adapter: Integrates with email, storage, payment providers, or external APIs.

SRP is about reasons to change, not about creating one class for every method. Keep closely related behavior together and separate responsibilities that evolve independently.

4. O — Open/Closed Principle (OCP)

Definition

Software entities should be open for extension but closed for modification. You should be able to introduce a new behavior through a suitable extension point without repeatedly rewriting stable, already-tested code.

OCP does not mean existing code must never change. Requirements, bug fixes, and improved designs can require changes. The principle encourages isolating likely variation behind abstractions or composition.

Example: repeated conditional logic

public decimal CalculateDiscount(string type, decimal amount)
{
    if (type == "Regular") return amount * 0.05m;
    if (type == "Premium") return amount * 0.10m;
    if (type == "Corporate") return amount * 0.15m;

    return 0m;
}

For a small, stable set of cases, a conditional may be perfectly reasonable. If discount rules are expected to grow independently, repeated modification can become difficult to maintain.

Use a strategy abstraction

public interface IDiscountPolicy
{
    decimal Calculate(decimal amount);
}

public sealed class RegularDiscount : IDiscountPolicy
{
    public decimal Calculate(decimal amount) => amount * 0.05m;
}

public sealed class PremiumDiscount : IDiscountPolicy
{
    public decimal Calculate(decimal amount) => amount * 0.10m;
}

public sealed class CorporateDiscount : IDiscountPolicy
{
    public decimal Calculate(decimal amount) => amount * 0.15m;
}

public class DiscountCalculator
{
    public decimal Calculate(IDiscountPolicy policy, decimal amount)
        => policy.Calculate(amount);
}

A new discount policy can be added as another implementation. The calculator does not need to know every discount formula.

Registering services in ASP.NET Core

builder.Services.AddScoped<RegularDiscount>();
builder.Services.AddScoped<PremiumDiscount>();
builder.Services.AddScoped<CorporateDiscount>();

This registers the implementations, but a real application also needs a selection mechanism, such as a resolver, keyed services, or a policy registry. Registering several implementations alone does not tell the application which one to use.

Common OCP techniques

• Strategy pattern for interchangeable algorithms.
• Polymorphism where different types have valid substitutable behavior.
• Decorator pattern for adding behavior around an existing component.
• Handlers or events for workflows with independently registered reactions.
• Configuration-driven rules when variation is genuinely data-driven.

Do not introduce an interface for every conditional. If behavior is unlikely to vary and the conditional is clear, keeping it simple may be better.

5. L — Liskov Substitution Principle (LSP)

Definition

Objects of a subtype should be usable wherever objects of the base type are expected without changing the correctness of the program. An implementation must honor the expectations established by its abstraction.

Example: invalid inheritance behavior

A rectangle abstraction that promises independent width and height changes is not a good base type for a square implementation that changes both dimensions whenever either setter is used.

public class Rectangle
{
    public virtual int Width { get; set; }
    public virtual int Height { get; set; }
    public int Area => Width * Height;
}

public class Square : Rectangle
{
    public override int Width
    {
        get => base.Width;
        set { base.Width = value; base.Height = value; }
    }

    public override int Height
    {
        get => base.Height;
        set { base.Height = value; base.Width = value; }
    }
}

static void Configure(Rectangle shape)
{
    shape.Width = 10;
    shape.Height = 5;
    Console.WriteLine(shape.Area); // Caller expects 50
}

Passing a Square to Configure violates the caller's expectation that the dimensions can be changed independently.

Prefer a contract that expresses the shared behavior

public interface IShape
{
    int GetArea();
}

public sealed class RectangleShape : IShape
{
    public int Width { get; }
    public int Height { get; }

    public RectangleShape(int width, int height)
    {
        Width = width;
        Height = height;
    }

    public int GetArea() => Width * Height;
}

public sealed class SquareShape : IShape
{
    public int Side { get; }

    public SquareShape(int side) => Side = side;

    public int GetArea() => Side * Side;
}

Both types satisfy the same small contract without pretending to support identical mutation behavior.

LSP design checks

• A derived type should honor the base type's documented behavior.
• Do not throw NotSupportedException for operations the abstraction promises callers can use.
• Do not silently return fake or incomplete results to satisfy an interface.
• Preserve meaningful invariants and error expectations.
• Prefer composition or a smaller capability interface when inheritance creates awkward exceptions.

Inheritance is not justified only because two objects share a name or conceptual relationship. The subtype must be behaviorally compatible with the abstraction.

6. I — Interface Segregation Principle (ISP)

Definition

Clients should not be forced to depend on methods they do not use. Prefer small, cohesive interfaces describing a specific capability instead of one large interface containing unrelated operations.

Example: a broad interface

public interface IMultiFunctionDevice
{
    void Print(Document document);
    void Scan(Document document);
    void Fax(Document document);
    void Copy(Document document);
}

A basic printer would have to implement methods it cannot support, perhaps by throwing exceptions or leaving methods empty. That indicates the interface combines too many capabilities.

Split by capability

public interface IPrinter
{
    void Print(Document document);
}

public interface IScanner
{
    void Scan(Document document);
}

public sealed class BasicPrinter : IPrinter
{
    public void Print(Document document)
    {
        // Print document
    }
}

public sealed class OfficeDevice : IPrinter, IScanner
{
    public void Print(Document document) { }
    public void Scan(Document document) { }
}

ISP in a .NET application

public interface IEmployeeReader
{
    Task<Employee?> GetByIdAsync(int id, CancellationToken ct);
}

public interface IEmployeeWriter
{
    Task AddAsync(Employee employee, CancellationToken ct);
    Task UpdateAsync(Employee employee, CancellationToken ct);
}

public interface IEmployeeExporter
{
    Task<Stream> ExportAsync(CancellationToken ct);
}

A read-only reporting component can depend on IEmployeeReader without receiving write operations. Split interfaces when consumers or responsibilities genuinely differ—not simply to make every interface tiny.

Interface Segregation is about the needs of consumers. A focused interface should describe a useful capability, not simply contain one method by default.

7. D — Dependency Inversion Principle (DIP)

Definition

High-level modules should not depend directly on low-level modules. Both should depend on abstractions. Abstractions should not depend on details; details should depend on abstractions.

High-level modules contain application or business policy. Low-level modules contain implementation details such as SQL access, email providers, file systems, and third-party APIs.

Example: direct dependency on infrastructure

public class OrderService
{
    private readonly SqlOrderRepository _repository;

    public OrderService()
    {
        _repository = new SqlOrderRepository();
    }

    public Task SaveAsync(Order order)
        => _repository.SaveAsync(order);
}

The high-level service creates and depends on a concrete persistence class. Replacing SQL persistence or testing the service independently becomes harder.

Depend on an abstraction

public interface IOrderRepository
{
    Task SaveAsync(Order order, CancellationToken ct);
}

public class SqlOrderRepository : IOrderRepository
{
    public Task SaveAsync(Order order, CancellationToken ct)
    {
        // Persist using EF Core or ADO.NET
        return Task.CompletedTask;
    }
}

public class OrderService
{
    private readonly IOrderRepository _repository;

    public OrderService(IOrderRepository repository)
    {
        _repository = repository;
    }

    public Task SaveAsync(Order order, CancellationToken ct)
        => _repository.SaveAsync(order, ct);
}

The service depends on IOrderRepository, not on a specific database implementation. The SQL repository implements the abstraction.

Dependency Injection in ASP.NET Core

Dependency Inversion is a design principle. Dependency Injection (DI) is a technique for supplying dependencies. ASP.NET Core includes a built-in DI container.

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddScoped<IOrderRepository,
    SqlOrderRepository>();

builder.Services.AddScoped<OrderService>();

var app = builder.Build();

app.MapPost("/orders", async (
    Order order,
    OrderService service,
    CancellationToken ct) =>
{
    await service.SaveAsync(order, ct);
    return Results.Accepted();
});

app.Run();

This is a minimal illustration. A real application must configure the database provider and DbContext, validate requests, and return appropriate responses for the use case.

Dependency Inversion vs Dependency Injection

Dependency Inversion Principle	Dependency Injection
A design principle about the direction of source-code dependencies.	A technique for providing an object with the dependencies it needs.
Encourages high-level policy and low-level details to depend on abstractions.	Can supply implementations through constructors, methods, or properties.
Can be applied with or without a DI container.	Can help implement DIP, but DI alone does not guarantee good design.

Understand service lifetimes

• Transient: A new instance is created each time the service is requested.
• Scoped: One instance is created per scope. In a typical ASP.NET Core web request, the request is a scope.
• Singleton: One instance is shared for the lifetime of the application container.

Do not inject a scoped service such as a typical EF Core DbContext directly into a singleton. A singleton must not capture a shorter-lived dependency in a way that causes lifetime or concurrency problems.

8. How the five principles work together

Principle	Design question	Practical result
SRP	Does this component have one cohesive responsibility?	Changes are localized to the part that owns the behavior.
OCP	Can likely variations be added without repeatedly changing stable policy code?	New strategies or handlers can be introduced through extension points.
LSP	Can every implementation honor the contract expected by callers?	Polymorphism remains predictable.
ISP	Does each consumer depend only on capabilities it needs?	Interfaces remain focused.
DIP	Does business policy depend on abstractions rather than infrastructure details?	Implementations can be replaced and tested more easily.

For example, an order workflow can have a focused application service (SRP), use an interchangeable pricing policy (OCP), rely on implementations that honor their contracts (LSP), expose small repository and notification capabilities (ISP), and receive those dependencies through constructor injection (DIP).

9. Applying SOLID in ASP.NET Core

SOLID can guide the organization of a Web API without forcing every project into a large enterprise architecture. A simple layered structure may be enough for many applications.

Example project structure

EmployeeManagement/
|
+-- EmployeeManagement.Api/
|   +-- Controllers/
|   +-- Contracts/
|   +-- Middleware/
|   +-- Program.cs
|
+-- EmployeeManagement.Application/
|   +-- Employees/
|   +-- Abstractions/
|   +-- Services/
|
+-- EmployeeManagement.Domain/
|   +-- Entities/
|   +-- ValueObjects/
|   +-- DomainServices/
|
+-- EmployeeManagement.Infrastructure/
    +-- Persistence/
    +-- Repositories/
    +-- ExternalServices/

This is one possible organization, not a mandatory folder structure. A smaller application may not need separate projects for every layer. Organize code around responsibilities and change boundaries rather than copying a template without understanding it.

Keep controllers focused on HTTP

[ApiController]
[Route("api/[controller]")]
public class EmployeesController : ControllerBase
{
    private readonly IEmployeeService _service;

    public EmployeesController(IEmployeeService service)
    {
        _service = service;
    }

    [HttpGet("{id:int}")]
    public async Task<ActionResult<EmployeeDto>> GetById(
        int id, CancellationToken ct)
    {
        var employee = await _service.GetByIdAsync(id, ct);

        if (employee is null)
            return NotFound();

        return Ok(employee);
    }
}

The controller handles HTTP input and output. It delegates the application operation to a service. The service should not need to know about MVC action results or HTTP response formatting.

Use abstractions at meaningful boundaries

Interfaces are useful where the application crosses a boundary: database access, external APIs, messaging, file storage, or a policy with multiple implementations. For a simple internal helper with one stable implementation, a concrete class may be sufficient.

Keep business rules independent of infrastructure

Entity Framework Core, Dapper, HTTP clients, and cloud SDKs are implementation technologies. Business rules should not become dependent on incidental details of those libraries unless the design intentionally chooses that trade-off.

Use asynchronous APIs appropriately

For I/O-bound work such as database calls and HTTP requests, use asynchronous APIs and pass CancellationToken through the call chain where appropriate. Async programming is not itself a SOLID principle, but it supports responsive and scalable application behavior.

10. SOLID and testing

A design that separates business behavior from infrastructure is often easier to test. Tests can provide controlled implementations of abstractions and verify one component without requiring a live database or external provider.

Example: fake dependency

public sealed class FakeOrderRepository : IOrderRepository
{
    public Order? SavedOrder { get; private set; }

    public Task SaveAsync(Order order, CancellationToken ct)
    {
        SavedOrder = order;
        return Task.CompletedTask;
    }
}

[Fact]
public async Task SaveAsync_StoresTheProvidedOrder()
{
    var repository = new FakeOrderRepository();
    var service = new OrderService(repository);
    var order = new Order();

    await service.SaveAsync(order, CancellationToken.None);

    Assert.Same(order, repository.SavedOrder);
}

This test illustrates the value of depending on a contract. The fake repository can be replaced by a mock or another test double when appropriate. Tests should verify meaningful behavior, not merely that a method was called.

• Unit tests: Validate focused business behavior and edge cases.
• Integration tests: Verify collaboration between components such as the API, database provider, and serialization pipeline.
• Contract tests: Verify that an implementation behaves according to an agreed interface or external API contract where that risk matters.

SOLID does not mean every class needs a mock-based unit test. Use integration tests for behavior that depends on real framework or database semantics.

11. Common misconceptions and overengineering

Every class must have an interface

False. Interfaces are valuable at useful boundaries and where multiple implementations, substitution, or independent testing matter. A simple class with one stable implementation may not need an interface.

SRP means one method per class

False. A class can contain several methods that support one cohesive responsibility. The principle concerns reasons to change, not method count.

OCP means never modifying existing code

False. Software evolves. OCP encourages extension points for likely variation; it does not forbid fixing or improving existing code.

Inheritance is always better than composition

False. Inheritance creates a behavioral contract. Composition is often more flexible when a component needs to collaborate with another capability without claiming to be a subtype.

Dependency Injection automatically means DIP

False. A class can receive a concrete infrastructure class through DI and still be tightly coupled to that detail. Dependency direction and abstraction boundaries matter.

More layers mean better architecture

False. Extra layers can add indirection and maintenance cost. Add boundaries when they protect meaningful responsibilities or isolate a volatile dependency.

Warning signs of overengineering

• Interfaces with one implementation and no meaningful boundary, created only by habit.
• Factories, managers, providers, and wrappers that merely forward calls without adding value.
• Generic repositories that hide important database capabilities or make queries harder to express.
• Many tiny classes that force readers to jump through files to understand one simple operation.
• Abstractions designed for hypothetical future requirements rather than plausible change.

12. Practical refactoring walkthrough

Imagine a single class in an ASP.NET Core application that creates an employee, validates the request, saves it with EF Core, sends a welcome email, and writes a file to disk. Refactor incrementally instead of rewriting the whole application.

Step 1: Identify responsibilities

• Employee creation rules belong to application or domain behavior.
• Database persistence belongs to the persistence implementation.
• Email delivery belongs to a notification adapter.
• File storage belongs to a storage adapter.
• HTTP validation and response mapping belong at the API boundary.

Step 2: Define useful contracts

public interface IEmployeeRepository
{
    Task AddAsync(Employee employee, CancellationToken ct);
}

public interface IWelcomeEmailSender
{
    Task SendAsync(Employee employee, CancellationToken ct);
}

public interface IEmployeeDocumentStore
{
    Task StoreAsync(Employee employee, Stream document,
        CancellationToken ct);
}

Step 3: Create an application service

public class CreateEmployeeService
{
    private readonly IEmployeeRepository _repository;
    private readonly IWelcomeEmailSender _emailSender;

    public CreateEmployeeService(
        IEmployeeRepository repository,
        IWelcomeEmailSender emailSender)
    {
        _repository = repository;
        _emailSender = emailSender;
    }

    public async Task CreateAsync(Employee employee, CancellationToken ct)
    {
        // Apply use-case and domain rules here.
        await _repository.AddAsync(employee, ct);
        await _emailSender.SendAsync(employee, ct);
    }
}

This is an illustrative flow. Production code should consider transaction boundaries, reliable delivery, duplicate requests, and failure handling. Saving an employee and sending an email are not one atomic database transaction. A transactional outbox may be appropriate when notification delivery must be reliable.

Step 4: Register implementations

builder.Services.AddScoped<IEmployeeRepository,
    EfEmployeeRepository>();

builder.Services.AddScoped<IWelcomeEmailSender,
    SmtpWelcomeEmailSender>();

builder.Services.AddScoped<CreateEmployeeService>();

Step 5: Test at the right boundaries

• Test employee creation rules without external services.
• Test the EF Core repository against a suitable relational test database when provider behavior matters.
• Test email adapter configuration and failure behavior separately.
• Test the API endpoint for validation, authorization, and HTTP response behavior.

13. Design review checklist

Use these questions when reviewing a class, service, or pull request:

• Does this class have a clear, cohesive responsibility?
• Are unrelated reasons to change mixed together?
• Are likely variations isolated behind an appropriate extension point?
• Do implementations honor the behavior promised by their abstractions?
• Are consumers forced to depend on methods they do not need?
• Does business policy depend directly on infrastructure details?
• Can important business behavior be tested without unnecessary external setup?
• Are interfaces and layers providing real value rather than adding indirection?
• Are asynchronous operations and cancellation handled consistently where appropriate?
• Are transaction and failure boundaries explicit when multiple systems are involved?

14. Practice roadmap

Build a small application and improve it step by step. Do not try to apply all five principles to every class at once.

Stage	Practice	Learning outcome
Beginner	Identify responsibilities in a controller and service. Separate HTTP, business, and persistence concerns.	Understand SRP and cohesion.
Intermediate	Replace growing conditional behavior with a strategy where variation is real. Create focused capability interfaces.	Understand OCP and ISP.
Intermediate	Review inheritance hierarchies and replace invalid ones with composition or smaller contracts.	Understand LSP.
Advanced	Invert dependencies around database, messaging, and external API boundaries. Register implementations in ASP.NET Core DI.	Understand DIP and composition roots.
Production-ready	Add unit and integration tests; review transactions, retries, failure handling, and deployment boundaries.	Apply principles to maintainable systems.

15. Important SOLID interview questions

1. What are SOLID principles?

SOLID is a set of five object-oriented design principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion.

2. What is SRP?

A class or module should have one cohesive responsibility and one primary reason to change.

3. What is OCP?

Software entities should allow behavior to be extended without repeatedly modifying stable code. Abstractions and composition can provide extension points.

4. What is LSP?

A subtype should be usable wherever its base abstraction is expected without violating the behavior callers rely on.

5. What is ISP?

Clients should not be forced to depend on methods they do not use. Prefer focused interfaces based on consumer needs.

6. What is DIP?

High-level policy and low-level implementation details should depend on abstractions rather than directly depending on one another.

7. What is the difference between DIP and DI?

DIP is a design principle about dependency direction. DI is a technique for supplying dependencies to an object. DI can help implement DIP but does not guarantee it.

8. How does ASP.NET Core support DI?

ASP.NET Core provides a built-in service container. Services are registered in startup configuration and injected, commonly through constructors or supported endpoint parameters.

9. Does every class need an interface?

No. Use interfaces where they provide a meaningful abstraction, such as a boundary, interchangeable implementation, or independently testable dependency.

10. Can SOLID be applied without design patterns?

Yes. Design patterns can help implement some principles, but SOLID does not require a particular pattern or architecture.

11. Is a repository pattern mandatory with Entity Framework Core?

No. EF Core's DbContext already provides unit-of-work behavior and DbSet provides repository-like access. A custom repository can be useful for a deliberate abstraction, but it should not be added automatically.

12. How do SOLID principles improve testability?

Focused responsibilities and abstractions make it easier to isolate business behavior, substitute dependencies, and test components at appropriate boundaries.

16. Official resources and further reading

SOLID is language-independent. Use official C# and .NET documentation to understand the language and framework features used in the examples.

• Microsoft Learn: Interfaces in C#
• Microsoft Learn: Dependency Injection in .NET
• Microsoft Learn: Dependency Injection in ASP.NET Core
• Microsoft: Modern Web Applications with ASP.NET Core
• Microsoft Learn: .NET Testing
• Microsoft Learn: C# Documentation

Final thoughts

SOLID principles help developers think carefully about responsibility, extension, substitutability, interfaces, and dependency direction. Their value is not measured by the number of interfaces or projects in a solution. Their value is measured by whether the resulting code is easier to understand, test, change, and operate.

As a .NET developer, practise these principles in controllers, application services, domain logic, repositories, and external integrations. Start with a real design problem, make the smallest useful improvement, and verify that the new structure genuinely reduces coupling or clarifies responsibility.

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