SOLID is a set of five object-oriented design principles that can help you spot tightly coupled code, unclear responsibilities, and awkward extension points in a C# application. Treat them as questions to guide small design decisions—not as rules to apply mechanically. The practical starting point is to understand what each principle asks, then refactor where a real change, testing need, or responsibility boundary calls for it.
What are the SOLID principles in C#?
SOLID names five principles: Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. They are not C# language features; they are design guidance for organizing object-oriented code.
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| 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 a subtype or 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 actually needs? |
| D | Dependency Inversion Principle (DIP) | Do higher-level policies depend on abstractions rather than implementation details? |
A Microsoft-published C# article expands O as “Open for extension and closed for modification” and lists “Dependency injection” for D. In current .NET architecture guidance, however, Dependency Inversion is the principle; Dependency Injection is a technique that can help implement it. That distinction matters when learning the acronym and applying the design.
How do you use each principle in C#?
Use the principles as diagnostic prompts. The right design depends on the code’s responsibilities, expected changes, and clients; there is no requirement to introduce an interface or abstraction for every class.
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Single Responsibility: look for unrelated reasons to change
A class is easier to reason about when its work forms one coherent responsibility. If one class handles business decisions, persistence, and formatting, changes to any of those concerns may force edits in the same place. Consider separating the responsibilities when they change for different reasons or need to be tested independently. A class with several methods is not automatically a violation; coherence matters more than method count.
Open-Closed: make likely changes local
Stable code should not need repeated edits every time a likely variation is added. For example, if an application must support different ways of sending a notification, a shared abstraction with distinct implementations may let the application select a new behavior without rewriting the business policy. The extension point should address a real variation; adding layers for hypothetical future changes can make code harder to follow.
Liskov Substitution: preserve the abstraction’s promises
An implementation should honor the expectations expressed by its abstraction. If code accepts a base type or interface, substituting another implementation should not unexpectedly break its assumptions, such as whether an operation is supported or what result it returns. When an implementation cannot meet those expectations, revise the abstraction or model the behavior explicitly rather than relying on inheritance or an interface name alone.
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A class should not have to depend on unrelated members just because they share a broad interface. If different callers use distinct parts of a contract, focused interfaces can reduce unnecessary coupling and make implementations clearer. Split an interface when clients genuinely have different needs—not simply to maximize the number of interfaces.
Dependency Inversion: point policy toward abstractions
Higher-level code—such as a business service—should depend on an abstraction for a changeable detail, such as storage or messaging, rather than directly depending on a particular implementation. In Microsoft’s .NET architecture guidance, the compile-time dependency points toward the abstraction. At runtime, that abstraction can be fulfilled by a selected implementation. The runtime call flow and compile-time dependency direction are not necessarily the same.
What is the difference between dependency inversion and dependency injection?
Dependency Inversion (DIP) is a design principle about the direction of dependencies: policy code relies on abstractions rather than concrete details. Dependency Injection (DI) is a technique for supplying an object’s dependencies from outside that object, often through its constructor. DI can make a DIP-based design practical, but using a container does not by itself make a design follow SOLID.
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In .NET, the built-in service container supports a common flow: define an abstraction, register an implementation, and inject the abstraction into the constructor of the class that needs it. The container then creates registered services and manages disposal according to their configured lifetimes.
A small .NET example
Here, WelcomeService depends on IMessageWriter, not on a particular writer. The application registers the implementation, and the service receives it through constructor injection:
public interface IMessageWriter
{
void Write(string message);
}
public sealed class ConsoleMessageWriter : IMessageWriter
{
public void Write(string message) => Console.WriteLine(message);
}
public sealed class WelcomeService
{
private readonly IMessageWriter _writer;
public WelcomeService(IMessageWriter writer) => _writer = writer;
public void Welcome(string name) => _writer.Write($"Welcome, {name}!");
}
var services = new ServiceCollection();
services.AddTransient<IMessageWriter, ConsoleMessageWriter>();
services.AddTransient<WelcomeService>();The abstraction gives the higher-level service a stable contract. A different implementation can be registered when the application needs different behavior, and a test can provide a suitable implementation without making the service construct its own infrastructure dependency. This is useful when the boundary is real; it is not a reason to wrap every concrete class in an interface.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How can you apply SOLID without overengineering?
Start with a concrete pressure in the code rather than aiming for a particular number of classes, interfaces, or injected services. Microsoft’s .NET dependency injection guidelines say that a class with many injected dependencies “might be a sign” that it has too many responsibilities and violates SRP. The wording is deliberately qualified: dependency count is a reason to inspect the class, not proof that it must be split.
- Identify the change, test difficulty, or responsibility boundary you are trying to address.
- Choose the smallest refactoring that isolates that concern and keeps dependencies explicit.
- Check that the new abstraction reflects a useful contract for a client, not just a desire to use a DI container.
- Keep services small, well-factored, and straightforward to test, while preserving a design that is easy for the team to understand.
Microsoft’s architecture overview explains dependency inversion and its relationship to dependency injection in Architectural principles. For the container, registration, and constructor-injection flow, see Dependency injection in .NET; its guidelines discuss service design and interpreting dependency counts.
Where should you learn more?
If you are new to C#, begin with Microsoft’s C# learning resources, which direct learners to material suited to different experience levels. For a book-length treatment with practical C# examples, patterns, SOLID, unit testing, and refactoring, Microsoft Press describes Adaptive Code: Agile coding with design patterns and SOLID principles, 2nd Edition. It is an optional deeper study, not a prerequisite for using the principles.
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