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SOLID is a set of five object-oriented design principles for managing responsibilities, variation, contracts, client needs, and dependencies in C#. The practical goal is not to create the largest possible network of interfaces and classes; it is to make likely changes local and keep the promises between components clear. Each principle can help when it addresses a real design pressure, and each can add needless indirection when it does not.
What SOLID means in C#
The acronym names five related principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. They are design guidance, not compiler rules or a required architecture. A class can compile and still be difficult to change; following a principle mechanically can also make a small, stable program harder to understand.
Use the principles to ask concrete questions: Are unrelated changes colliding in one type? Is a stable policy repeatedly edited for each expected variation? Does a substitute honor the behavior callers rely on? Do clients depend on operations they do not use? Does high-level policy know details it should not need to know?
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Microsoft Learn describes separation of concerns and dependency inversion as architectural principles for .NET applications. Its guidance notes that dependency inversion can reverse compile-time dependencies while runtime calls still flow from the higher-level component to the lower-level one. It also states: “The practice of dependency injection is made possible by following the dependency inversion principle.” Microsoft Learn: Architectural principles – .NET.
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1. Single Responsibility Principle: group code by one coherent reason to change
SRP says a type should have one responsibility and one reason to change. It does not prescribe one method per class, a maximum class size, or a rule that every operation needs its own type. The useful question is whether a type combines concerns that are likely to change independently.
Example: separate order calculation from persistence
Suppose an order service calculates a total and writes it to a database. Tax or discount rules may change for business reasons, while the database mechanism may change for infrastructure reasons. Putting both in one type couples those change paths.
public sealed class OrderService
{
private readonly IOrderStore _store;
public OrderService(IOrderStore store) => _store = store;
public decimal CalculateTotal(Order order) =>
order.Subtotal + order.Tax;
public void Save(Order order) => _store.Save(order);
}
public interface IOrderStore
{
void Save(Order order);
}This example still has two operations in one service, but calculation and persistence are distinguishable responsibilities. If calculation becomes a substantial policy with its own rules and tests, move it into a calculator; if the formula is trivial and stable, a separate class may add ceremony without value. The separation is justified when the two concerns change independently, not merely because they can be named separately.
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2. Open/Closed Principle: extend expected variation without rewriting stable policy
OCP is commonly expressed as “open for extension, closed for modification.” In practice, it means that when a behavior family is expected to grow, the stable policy can work through a contract while new implementations supply the variation. It does not mean every conditional is a design flaw. A short switch over a small, closed set of cases may be clearer and safer than a hierarchy that nobody needs.
Example: support multiple payment methods
Assume a checkout application is expected to add card and bank-transfer processing, with each processor implementing the same operation. A strategy interface makes that variation explicit:
public interface IPaymentProcessor
{
void Charge(decimal amount);
}
public sealed class CardPaymentProcessor : IPaymentProcessor
{
public void Charge(decimal amount)
{
// Call the card provider.
}
}
public sealed class BankTransferProcessor : IPaymentProcessor
{
public void Charge(decimal amount)
{
// Initiate a bank transfer.
}
}
public sealed class CheckoutService
{
private readonly IPaymentProcessor _processor;
public CheckoutService(IPaymentProcessor processor) =>
_processor = processor;
public void Complete(decimal total) => _processor.Charge(total);
}The stable checkout flow no longer needs an edit for each processor; the application still needs a deliberate way to select and supply the right implementation. The abstraction is worthwhile if payment methods are genuine expected alternatives. For one permanent method, the interface and selection machinery may be needless.
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Where patterns fit
Strategy is a pattern that can represent interchangeable behavior, as in the payment example. It is an example of an OCP-friendly design, not a requirement of OCP. A factory can centralize construction or selection when that policy is meaningful; adding one solely to instantiate a single obvious object creates another place to navigate.
3. Liskov Substitution Principle: replacements must keep the contract
LSP concerns behavior, not just type compatibility. A derived class or interface implementation may compile yet violate the assumptions callers rely on. A substitute should not reject inputs that the contract permits, promise less than the contract guarantees, or introduce surprising behavior.
Example: a read-only account is not a writable account
Suppose callers rely on every account being able to accept a deposit. A frozen account subtype that throws for valid deposits violates that promise:
public class Account
{
public virtual decimal Balance { get; protected set; }
public virtual void Deposit(decimal amount)
{
if (amount < 0) throw new ArgumentOutOfRangeException(nameof(amount));
Balance += amount;
}
}
public sealed class FrozenAccount : Account
{
public override void Deposit(decimal amount) =>
throw new InvalidOperationException("Account is frozen.");
}Any caller that receives an Account and valid nonnegative amount can reasonably expect the base operation to work; FrozenAccount breaks that expectation. One repair is to model distinct capabilities instead of promising a behavior that some accounts cannot provide:
public interface IAccount
{
decimal Balance { get; }
}
public interface IDepositableAccount : IAccount
{
void Deposit(decimal amount);
}Callers that need to deposit can require IDepositableAccount; read-only consumers can use IAccount. This extra distinction is useful only if the domain really has those different capabilities. If all accounts satisfy the same deposit contract, keep the simpler contract and implement it consistently.
4. Interface Segregation Principle: give each client the operations it needs
ISP advises against forcing a client or implementation to depend on irrelevant operations. A broad interface can make small clients carry methods they never call and can make implementers provide meaningless stubs.
Example: split worker capabilities by client need
Imagine a worker interface combines printing, scanning, and faxing, but some devices or clients need only one capability:
public interface IOfficeMachine
{
void Print(Document document);
void Scan(Document document);
void Fax(Document document);
}A device without fax support may be forced to throw or silently do nothing. Focused interfaces align the contract with actual use:
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public interface IPrinter
{
void Print(Document document);
}
public interface IScanner
{
void Scan(Document document);
}
public interface IFax
{
void Fax(Document document);
}
public sealed class PrintJob
{
private readonly IPrinter _printer;
public PrintJob(IPrinter printer) => _printer = printer;
public void Run(Document document) => _printer.Print(document);
}PrintJob depends only on printing; a multifunction device can implement several capabilities. Do not split every method into its own interface by default. If all clients need the same cohesive set of operations and implementations support them, one interface may be simpler and more expressive.
5. Dependency Inversion Principle: keep policy independent of implementation details
DIP says high-level policy should depend on abstractions rather than concrete low-level details. The abstraction belongs at the boundary needed by the policy; an interface is not automatically valuable merely because a dependency-injection container can register it.
Example: application policy should not construct a database client
If an order application service directly creates a particular database client, its business flow is tied to that infrastructure choice:
public sealed class OrderApplicationService
{
public void Save(Order order)
{
var database = new SqlOrderDatabase();
database.Save(order);
}
}Instead, let the application policy depend on an operation it needs, and let infrastructure implement it:
public interface IOrderStore
{
void Save(Order order);
}
public sealed class OrderApplicationService
{
private readonly IOrderStore _store;
public OrderApplicationService(IOrderStore store) => _store = store;
public void Save(Order order) => _store.Save(order);
}
public sealed class SqlOrderStore : IOrderStore
{
public void Save(Order order)
{
// Persist the order with the chosen database technology.
}
}The service can be tested with a substitute store, and the database-specific implementation can change without rewriting the service’s policy. The runtime call still reaches the store; what changes is the compile-time dependency direction. In a .NET application, dependency injection is a common technique for supplying SqlOrderStore to the service, but dependency injection is not another name for DIP. A direct concrete dependency can remain reasonable in a small, stable program with no meaningful substitution or testing boundary.
Patterns at dependency boundaries
An Adapter can translate an external API into an application-owned interface, isolating the application from the provider’s types and conventions. A Decorator can wrap an abstraction to add behavior such as logging or caching without modifying the wrapped implementation. Both are options when a real boundary or cross-cutting need exists; they are not mandatory components of every SOLID design.
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A non-trivial business application often separates presentation, application policy, domain rules, and infrastructure into logical areas. This can clarify responsibility and keep infrastructure choices from spreading into business policy, but SOLID does not mandate Clean Architecture, a fixed number of layers, repositories, or a particular DI container. Microsoft Learn’s overview of common .NET web architectures describes layered approaches and their trade-offs. Microsoft Learn: Common web application architectures – .NET.
For example, an application service can coordinate an order workflow, call a domain calculation, and save through an interface owned by the application or domain boundary. An infrastructure component can implement that interface using a database or external service. This arrangement is useful when the boundary protects meaningful policy from volatile details; it is overhead if it only renames a simple call without making change, substitution, or testing clearer.
Microsoft’s C#-example download on application partitioning and layering is another reference for the architectural context, though it should not be treated as a mandate for one universal layer structure. Microsoft Download Center: Partitioning and Layering a Software Application (Examples in C#).
How to decide whether a SOLID refactor is worthwhile
Before adding an interface, class, or pattern, compare the current and proposed design against the actual pressure in the code:
- Expected changes: Which likely change becomes localized, and which types still need editing?
- Contracts: What inputs and behaviors do callers rely on, and can every implementation honor them?
- Client needs: Does each client depend only on operations it uses?
- Substitution and testing: Does the boundary make a realistic alternate implementation or focused test easier?
- Dependency direction: Does policy depend on a stable abstraction, or does it know about details that may change?
- Structural cost: Are extra types and indirection making the design harder to follow than the change they protect against?
SOLID offers no guarantee that more abstractions produce fewer defects or faster development in every codebase. Prefer the smallest design that keeps real change pressure manageable, and revisit the design when requirements expose a new variation or boundary. The book Adaptive Code: Agile coding with design patterns and SOLID principles is a publisher-listed further reading option on these topics.
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