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SOLID is a set of five object-oriented design principles that can help C# developers keep code easier to change and test. They are guidance, not rules to apply mechanically: use them to spot design pressure, then make the smallest change that clarifies responsibilities or isolates a real point of change.
What are the SOLID principles in C#?
SOLID is an acronym for five principles: Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. A Microsoft-published C# article uses “Dependency injection” when expanding the D, but .NET architecture guidance distinguishes the underlying principle—Dependency Inversion—from dependency injection, a technique that can help implement it.
| Letter | Principle | Question to ask |
|---|---|---|
| S | Single Responsibility Principle (SRP) | Does this class have one coherent responsibility, or are unrelated reasons for change accumulating in it? |
| O | Open-Closed Principle (OCP) | Can likely new behavior be added through an appropriate extension point without repeatedly changing stable code? |
| L | Liskov Substitution Principle (LSP) | Can an implementation stand in for its abstraction while preserving the expectations of code that uses it? |
| I | Interface Segregation Principle (ISP) | Does each client depend only on the interface members it needs? |
| D | Dependency Inversion Principle (DIP) | Does higher-level policy depend on abstractions rather than details such as a particular storage implementation? |
These questions are prompts for design conversations, not tests with one universally correct answer. The right design depends on what the code does and what is likely to change.
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How do you apply SOLID principles without overengineering?
Start with a real source of friction: a class that changes for unrelated reasons, a new feature that requires edits across stable code, or a dependency that makes a useful test difficult. Then consider whether a small refactor would make that pressure easier to manage.
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- Keep responsibilities coherent, but do not split every method or class merely to reduce its size.
- Create abstractions where they isolate a meaningful boundary, likely variation, or concrete testing need—not automatically for every class.
- Design interfaces around what their clients use rather than forcing clients to depend on unrelated members.
- Check that derived types and implementations honor the expectations of the abstractions they claim to fulfill.
- Prefer explicit dependencies and services that are small, well-factored, and straightforward to test.
Microsoft’s .NET dependency injection guidelines caution that many injected dependencies “might be a sign” that a class has too many responsibilities. That is a reason to inspect the class, not a numeric threshold or proof of an SRP violation. A class may legitimately coordinate several collaborators; look at whether its responsibilities belong together.
What is the difference between Dependency Inversion and Dependency Injection?
Dependency Inversion is a design principle
DIP concerns the direction of compile-time dependencies. Higher-level code should depend on abstractions rather than on concrete implementation details. For example, application logic can depend on an interface for writing messages instead of directly depending on a particular messaging system.
Dependency Injection is a technique
DI supplies an object’s dependencies from outside that object, commonly through its constructor. In .NET, the built-in service container can create objects and provide their registered dependencies. DI is one way to support DIP; using a container by itself does not make a design SOLID.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThe distinction is easiest to see across two moments: when code is compiled, a higher-level service refers to an abstraction; when the application runs, that abstraction is fulfilled by a selected implementation. The runtime implementation can differ without making the higher-level source code depend directly on its details.
How does dependency injection work in a small C# example?
Suppose a notification service needs to write a message. It can depend on an interface, while a separate class provides the implementation. Register the implementation during application setup, then let .NET pass it to the service’s constructor.
public interface IMessageWriter
{
void Write(string message);
}
public sealed class ConsoleMessageWriter : IMessageWriter
{
public void Write(string message) => Console.WriteLine(message);
}
public sealed class NotificationService
{
private readonly IMessageWriter _writer;
public NotificationService(IMessageWriter writer)
{
_writer = writer;
}
public void Send(string message) => _writer.Write(message);
}
// In application setup:
services.AddTransient<IMessageWriter, ConsoleMessageWriter>();
services.AddTransient<NotificationService>();Here, NotificationService depends on IMessageWriter, not on ConsoleMessageWriter. The registration tells the .NET service collection which implementation to provide. When a consumer requests NotificationService, the container can construct it and supply the registered writer. The container also manages disposal according to the registered service lifetime and the applicable .NET lifetime rules.
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Microsoft documents the practical sequence as defining an abstraction, registering an implementation, and injecting it into the constructor of the class that needs it. Choose a service lifetime to match the service’s behavior and application needs; registration is configuration, not a substitute for deciding whether the dependency belongs in the design.
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What is a practical first SOLID refactor?
- Find a concrete pressure. Choose a class that changes for unrelated reasons, is difficult to test because it creates its own external dependency, or needs repeated edits whenever a likely behavior is added.
- Name the responsibility or boundary. Describe what the class should own, or identify the implementation detail that should be replaceable. If the boundary is not meaningful, an interface may add ceremony without solving a problem.
- Make one focused change. For example, move message delivery behind an interface and pass the implementation into the consuming service. Avoid redesigning neighboring code unless the same pressure exists there.
- Check behavior and client expectations. Run the relevant tests, and verify that any replacement implementation still honors the contract its callers rely on.
- Reassess the result. The refactor should make responsibility, variation, or testing clearer. If it mainly adds indirection, reconsider whether the abstraction is useful.
Where can you learn more about C# and SOLID?
If you are new to C#, start with Microsoft’s C# learning resources, which direct learners to material suited to different experience levels. For the .NET implementation details, read Microsoft’s guidance on architectural principles, the dependency injection pattern, and dependency injection guidelines.
For a book-length treatment with practical C# examples, design patterns, SOLID, unit testing, and refactoring, see Microsoft Press’s description of Adaptive Code: Agile coding with design patterns and SOLID principles, 2nd Edition. It is an optional further-reading choice, not a prerequisite.
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