dotnet-solid-principles
SKILL.md
dotnet-solid-principles
Foundational design principles for .NET applications. Covers each SOLID principle with concrete C# anti-patterns and fixes, plus DRY guidance with nuance on when duplication is acceptable. These principles guide class design, interface contracts, and dependency management across all .NET project types.
Scope
- SOLID principles with C# anti-patterns and fixes
- DRY guidance and when duplication is acceptable
- SRP compliance tests and class design heuristics
- Interface segregation and dependency inversion patterns
Out of scope
- Architectural patterns (vertical slices, request pipelines, caching) -- see [skill:dotnet-architecture-patterns]
- DI container mechanics (registration, lifetimes, keyed services) -- see [skill:dotnet-csharp-dependency-injection]
- Code smells and anti-pattern detection -- see [skill:dotnet-csharp-code-smells]
Cross-references: [skill:dotnet-architecture-patterns] for clean architecture and vertical slices, [skill:dotnet-csharp-dependency-injection] for DI registration patterns and lifetime management, [skill:dotnet-csharp-code-smells] for anti-pattern detection, [skill:dotnet-csharp-coding-standards] for naming and style conventions.
Single Responsibility Principle (SRP)
A class should have only one reason to change. Apply the "describe in one sentence" test: if you cannot describe what a class does in one sentence without using "and" or "or", it likely violates SRP.
Anti-Pattern: God Class
// WRONG -- OrderService handles validation, persistence, email, and PDF generation
public class OrderService
{
private readonly AppDbContext _db;
private readonly SmtpClient _smtp;
public OrderService(AppDbContext db, SmtpClient smtp)
{
_db = db;
_smtp = smtp;
}
public async Task<Order> CreateOrderAsync(CreateOrderRequest request)
{
// Validation logic (reason to change #1)
if (string.IsNullOrEmpty(request.CustomerId))
throw new ArgumentException("Customer required");
// Persistence logic (reason to change #2)
var order = new Order { CustomerId = request.CustomerId };
_db.Orders.Add(order);
await _db.SaveChangesAsync();
// Email notification (reason to change #3)
var message = new MailMessage("noreply@shop.com", request.Email,
"Order Confirmed", $"Order {order.Id} created.");
await _smtp.SendMailAsync(message);
// PDF generation (reason to change #4)
GenerateInvoicePdf(order);
return order;
}
private void GenerateInvoicePdf(Order order) { /* ... */ }
}
```text
### Fix: Separate Responsibilities
```csharp
// Each class has one reason to change
public sealed class OrderCreator(
IOrderValidator validator,
IOrderRepository repository,
IOrderNotifier notifier)
{
public async Task<Order> CreateAsync(
CreateOrderRequest request, CancellationToken ct)
{
validator.Validate(request);
var order = await repository.AddAsync(request, ct);
await notifier.OrderCreatedAsync(order, ct);
return order;
}
}
public sealed class OrderValidator : IOrderValidator
{
public void Validate(CreateOrderRequest request)
{
ArgumentException.ThrowIfNullOrEmpty(request.CustomerId);
// ... validation rules
}
}
public sealed class OrderRepository(AppDbContext db) : IOrderRepository
{
public async Task<Order> AddAsync(
CreateOrderRequest request, CancellationToken ct)
{
var order = new Order { CustomerId = request.CustomerId };
db.Orders.Add(order);
await db.SaveChangesAsync(ct);
return order;
}
}
```text
### Anti-Pattern: Fat Controller
```csharp
// WRONG -- controller contains business logic, mapping, and persistence
app.MapPost("/api/orders", async (
CreateOrderRequest request,
AppDbContext db,
ILogger<Program> logger) =>
{
// Validation in the endpoint
if (request.Lines.Count == 0)
return Results.BadRequest("At least one line required");
// Business logic in the endpoint
var total = request.Lines.Sum(l => l.Quantity * l.Price);
if (total > 100_000)
return Results.BadRequest("Order exceeds credit limit");
// Mapping in the endpoint
var order = new Order
{
CustomerId = request.CustomerId,
Total = total,
Lines = request.Lines.Select(l => new OrderLine
{
ProductId = l.ProductId,
Quantity = l.Quantity,
Price = l.Price
}).ToList()
};
// Persistence in the endpoint
db.Orders.Add(order);
await db.SaveChangesAsync();
logger.LogInformation("Order {OrderId} created", order.Id);
return Results.Created($"/api/orders/{order.Id}", order);
});
```text
Move business logic to a handler; keep the endpoint thin:
```csharp
app.MapPost("/api/orders", async (
CreateOrderRequest request,
IOrderHandler handler,
CancellationToken ct) =>
{
var result = await handler.CreateAsync(request, ct);
return result switch
{
{ IsSuccess: true } => Results.Created(
$"/api/orders/{result.Value.Id}", result.Value),
_ => Results.ValidationProblem(result.Errors)
};
});
```text
---
## Open/Closed Principle (OCP)
Classes should be open for extension but closed for modification. Add new behavior by implementing new types, not by editing existing switch/if chains.
### Anti-Pattern: Switch on Type
```csharp
// WRONG -- adding a new discount type requires modifying this method
public decimal CalculateDiscount(Order order)
{
switch (order.DiscountType)
{
case "Percentage":
return order.Total * order.DiscountValue / 100;
case "FixedAmount":
return order.DiscountValue;
case "BuyOneGetOneFree":
return order.Lines
.Where(l => l.Quantity >= 2)
.Sum(l => l.Price);
default:
return 0;
}
}
```text
### Fix: Strategy Pattern
```csharp
public interface IDiscountStrategy
{
decimal Calculate(Order order);
}
public sealed class PercentageDiscount(decimal percentage) : IDiscountStrategy
{
public decimal Calculate(Order order) =>
order.Total * percentage / 100;
}
public sealed class FixedAmountDiscount(decimal amount) : IDiscountStrategy
{
public decimal Calculate(Order order) =>
Math.Min(amount, order.Total);
}
// New discount type -- no existing code modified
public sealed class BuyOneGetOneFreeDiscount : IDiscountStrategy
{
public decimal Calculate(Order order) =>
order.Lines
.Where(l => l.Quantity >= 2)
.Sum(l => l.Price);
}
// Usage -- resolved via DI or factory
public sealed class OrderPricing(
IEnumerable<IDiscountStrategy> strategies)
{
public decimal ApplyBestDiscount(Order order) =>
strategies.Max(s => s.Calculate(order));
}
```text
### Extension via Abstract Classes
When strategies share significant behavior, use an abstract base class:
```csharp
public abstract class NotificationSender
{
public async Task SendAsync(Notification notification, CancellationToken ct)
{
// Shared behavior: validation and logging
ArgumentNullException.ThrowIfNull(notification);
await SendCoreAsync(notification, ct);
}
protected abstract Task SendCoreAsync(
Notification notification, CancellationToken ct);
}
public sealed class EmailNotificationSender(IEmailClient client)
: NotificationSender
{
protected override async Task SendCoreAsync(
Notification notification, CancellationToken ct)
{
await client.SendEmailAsync(
notification.Recipient, notification.Subject,
notification.Body, ct);
}
}
```text
---
## Liskov Substitution Principle (LSP)
Subtypes must be substitutable for their base types without altering program correctness. A subclass must honor the behavioral contract of its parent -- preconditions cannot be strengthened, postconditions cannot be weakened.
### Anti-Pattern: Throwing in Override
```csharp
public class FileStorage : IStorage
{
public virtual Stream OpenRead(string path) =>
File.OpenRead(path);
}
// WRONG -- ReadOnlyFileStorage violates the base contract by
// throwing on a method the base type supports
public class ReadOnlyFileStorage : FileStorage
{
public override Stream OpenRead(string path)
{
if (!File.Exists(path))
throw new InvalidOperationException(
"Cannot open files in read-only mode");
return base.OpenRead(path);
}
// Surprise: callers expecting FileStorage behavior get exceptions
}
```text
### Anti-Pattern: Collection Covariance Pitfall
```csharp
// WRONG -- List<T> is not covariant; this compiles but causes runtime issues
IList<Animal> animals = new List<Dog>(); // Compile error (correctly)
// However, arrays ARE covariant in C# -- this compiles but throws at runtime:
Animal[] animals = new Dog[10];
animals[0] = new Cat(); // ArrayTypeMismatchException at runtime!
```csharp
### Fix: Use Covariant Interfaces
```csharp
// IEnumerable<out T> and IReadOnlyList<out T> are covariant
IEnumerable<Animal> animals = new List<Dog>(); // Safe -- read-only
IReadOnlyList<Animal> readOnlyAnimals = new List<Dog>(); // Safe
// When you need mutability, keep the concrete type
List<Dog> dogs = [new Dog("Rex"), new Dog("Buddy")];
ProcessAnimals(dogs); // Pass to covariant parameter
void ProcessAnimals(IReadOnlyList<Animal> animals)
{
foreach (var animal in animals)
animal.Speak();
}
```text
### LSP Compliance Checklist
- Derived classes do not throw new exception types that the base does not declare
- Overrides do not add preconditions (e.g., null checks the base does not require)
- Overrides do not weaken postconditions (e.g., returning null when base guarantees non-null)
- Behavioral contracts are preserved: if `ICollection.Add` succeeds on the base, it must succeed on the derived type
---
## Interface Segregation Principle (ISP)
Clients should not be forced to depend on methods they do not use. Prefer narrow, role-specific interfaces over wide "header" interfaces.
### Anti-Pattern: Header Interface
```csharp
// WRONG -- IWorker forces all implementations to support every capability
public interface IWorker
{
Task DoWorkAsync(CancellationToken ct);
void ClockIn();
void ClockOut();
Task<decimal> CalculatePayAsync();
void RequestTimeOff(DateRange range);
Task SubmitExpenseAsync(Expense expense);
}
// ContractWorker does not clock in/out or request time off
public class ContractWorker : IWorker
{
public Task DoWorkAsync(CancellationToken ct) => /* ... */;
public void ClockIn() => throw new NotSupportedException(); // ISP violation
public void ClockOut() => throw new NotSupportedException(); // ISP violation
public Task<decimal> CalculatePayAsync() => /* ... */;
public void RequestTimeOff(DateRange range) =>
throw new NotSupportedException(); // ISP violation
public Task SubmitExpenseAsync(Expense expense) =>
throw new NotSupportedException(); // ISP violation
}
```text
### Fix: Role Interfaces
```csharp
public interface IWorkPerformer
{
Task DoWorkAsync(CancellationToken ct);
}
public interface ITimeTrackable
{
void ClockIn();
void ClockOut();
}
public interface IPayable
{
Task<decimal> CalculatePayAsync();
}
public interface ITimeOffEligible
{
void RequestTimeOff(DateRange range);
}
// FullTimeEmployee implements all applicable interfaces
public sealed class FullTimeEmployee :
IWorkPerformer, ITimeTrackable, IPayable, ITimeOffEligible
{
public Task DoWorkAsync(CancellationToken ct) => /* ... */;
public void ClockIn() { /* ... */ }
public void ClockOut() { /* ... */ }
public Task<decimal> CalculatePayAsync() => /* ... */;
public void RequestTimeOff(DateRange range) { /* ... */ }
}
// ContractWorker only implements what it needs
public sealed class ContractWorker : IWorkPerformer, IPayable
{
public Task DoWorkAsync(CancellationToken ct) => /* ... */;
public Task<decimal> CalculatePayAsync() => /* ... */;
}
```text
### Practical .NET ISP
The .NET BCL demonstrates ISP well:
| Wide Interface | Segregated Alternatives |
|---|---|
| `IList<T>` (read + write) | `IReadOnlyList<T>` (read only) |
| `ICollection<T>` | `IReadOnlyCollection<T>` |
| `IDictionary<K,V>` | `IReadOnlyDictionary<K,V>` |
Accept the narrowest interface your method actually needs:
```csharp
// WRONG -- requires IList<T> but only reads
public decimal CalculateTotal(IList<OrderLine> lines) =>
lines.Sum(l => l.Price * l.Quantity);
// RIGHT -- accepts IReadOnlyList<T> since it only reads
public decimal CalculateTotal(IReadOnlyList<OrderLine> lines) =>
lines.Sum(l => l.Price * l.Quantity);
// BEST for iteration only -- accepts IEnumerable<T>
public decimal CalculateTotal(IEnumerable<OrderLine> lines) =>
lines.Sum(l => l.Price * l.Quantity);
```text
---
## Dependency Inversion Principle (DIP)
High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details.
### Anti-Pattern: Direct Dependency
```csharp
// WRONG -- high-level OrderProcessor depends directly on low-level SqlOrderRepository
public sealed class OrderProcessor
{
private readonly SqlOrderRepository _repository = new();
private readonly SmtpEmailSender _emailSender = new();
public async Task ProcessAsync(Order order)
{
await _repository.SaveAsync(order); // Tight coupling to SQL
await _emailSender.SendAsync(order.Email, // Tight coupling to SMTP
"Order processed", $"Order {order.Id}");
}
}
```text
### Fix: Depend on Abstractions
```csharp
public interface IOrderRepository
{
Task SaveAsync(Order order, CancellationToken ct = default);
Task<Order?> GetByIdAsync(string id, CancellationToken ct = default);
}
public interface INotificationService
{
Task NotifyAsync(string recipient, string subject,
string body, CancellationToken ct = default);
}
// High-level module depends on abstractions
public sealed class OrderProcessor(
IOrderRepository repository,
INotificationService notifier)
{
public async Task ProcessAsync(Order order, CancellationToken ct)
{
await repository.SaveAsync(order, ct);
await notifier.NotifyAsync(order.Email,
"Order processed", $"Order {order.Id}", ct);
}
}
// Low-level modules implement abstractions
public sealed class SqlOrderRepository(AppDbContext db) : IOrderRepository
{
public async Task SaveAsync(Order order, CancellationToken ct) =>
/* EF Core persistence */;
public async Task<Order?> GetByIdAsync(string id, CancellationToken ct) =>
await db.Orders.FindAsync([id], ct);
}
```text
### DI Registration
Register abstractions with Microsoft.Extensions.DependencyInjection. See [skill:dotnet-csharp-dependency-injection] for lifetime management, keyed services, and decoration patterns.
```csharp
builder.Services.AddScoped<IOrderRepository, SqlOrderRepository>();
builder.Services.AddScoped<INotificationService, SmtpNotificationService>();
builder.Services.AddScoped<OrderProcessor>();
```csharp
### DIP Boundaries
Apply DIP at module boundaries, not everywhere:
- **DO** abstract infrastructure (database, email, file system, HTTP clients)
- **DO** abstract cross-cutting concerns (logging is already abstracted via `ILogger<T>`)
- **DO NOT** abstract simple value objects, DTOs, or internal implementation details
- **DO NOT** create `IFoo`/`Foo` pairs for every class -- only abstract where substitution adds value (testing, multiple implementations, or anticipated change)
---
## DRY (Don't Repeat Yourself)
Every piece of knowledge should have a single, authoritative representation. But DRY is about knowledge duplication, not code duplication.
### When to Apply DRY
Apply DRY when two pieces of code represent the **same concept** and must change together:
```csharp
// WRONG -- tax rate duplicated across two services
public sealed class InvoiceService
{
public decimal CalculateTax(decimal amount) => amount * 0.08m;
}
public sealed class QuoteService
{
public decimal EstimateTax(decimal amount) => amount * 0.08m;
}
// RIGHT -- single source of truth
public static class TaxRates
{
public const decimal StandardRate = 0.08m;
}
```text
### Rule of Three
Do not abstract prematurely. Wait until you see the same pattern three times before extracting a shared abstraction:
1. **First occurrence** -- write it inline
2. **Second occurrence** -- note the duplication but keep it (the two usages may diverge)
3. **Third occurrence** -- extract a shared method, class, or utility
### When Duplication Is Acceptable
Not all code similarity represents knowledge duplication:
```csharp
// These look similar but represent DIFFERENT business concepts
// They will evolve independently -- DO NOT merge them
public sealed class CustomerValidator
{
public bool IsValid(Customer customer) =>
!string.IsNullOrEmpty(customer.Name) &&
!string.IsNullOrEmpty(customer.Email);
}
public sealed class SupplierValidator
{
public bool IsValid(Supplier supplier) =>
!string.IsNullOrEmpty(supplier.Name) &&
!string.IsNullOrEmpty(supplier.ContactEmail);
}
```text
**Acceptable duplication scenarios:**
- Test setup code that looks similar across test classes (coupling tests to shared helpers makes them fragile)
- DTOs for different API versions (V1 and V2 may share fields now but diverge later)
- Configuration for different environments (dev and prod configs that happen to be similar today)
- Mapping code between layers (coupling layers to share mappers defeats the purpose of separate layers)
### Abstracting Shared Behavior
When you do extract, prefer composition over inheritance:
```csharp
// Prefer: composition via a shared utility
public static class StringValidation
{
public static bool IsNonEmpty(string? value) =>
!string.IsNullOrWhiteSpace(value);
}
// Over: inheritance via a base class
// (couples validators to a shared base, harder to test independently)
```text
---
## Applying the Principles Together
### Decision Guide
| Symptom | Likely Violation | Fix |
|---|---|---|
| Class described with "and" | SRP | Split into focused classes |
| Modifying existing code to add features | OCP | Use strategy/plugin pattern |
| `NotSupportedException` in overrides | LSP | Redesign hierarchy or use composition |
| Empty/throwing interface methods | ISP | Split into role interfaces |
| `new` keyword for dependencies | DIP | Inject via constructor |
| Magic numbers/strings in multiple files | DRY | Extract constants or config |
| Copy-pasted code blocks (3+) | DRY | Extract shared method |
### SRP Compliance Test
For each class, answer these questions:
1. **One-sentence test:** Can you describe the class's purpose in one sentence without "and" or "or"?
2. **Change-reason test:** List all reasons this class might need to change. If more than one, consider splitting.
3. **Dependency count test:** Does the constructor take more than 3-4 dependencies? High parameter counts often signal multiple responsibilities.
---
## Agent Gotchas
1. **Do not create `IFoo`/`Foo` pairs for every class.** DIP calls for abstractions at module boundaries (infrastructure, external services), not for every internal class. Unnecessary interfaces add indirection without value and clutter the codebase.
2. **Do not merge similar-looking code from different bounded contexts.** Two validators or DTOs that look alike but serve different business concepts should remain separate. Premature DRY creates coupling between concepts that evolve independently.
3. **Do not use inheritance to share behavior between unrelated types.** Prefer composition (injecting a shared service or using extension methods) over inheriting from a common base class. Inheritance creates tight coupling and makes LSP violations more likely.
4. **Fat controllers and god classes are SRP violations.** When generating endpoint handlers, keep them thin -- delegate to dedicated services for validation, business logic, and persistence. Apply the "one sentence" test to each class.
5. **Switch statements on type discriminators violate OCP.** Replace them with polymorphism (strategy pattern, interface dispatch) so new types can be added without modifying existing code.
6. **Array covariance in C# is unsafe.** `Animal[] animals = new Dog[10]` compiles but throws `ArrayTypeMismatchException` at runtime when adding non-Dog elements. Use `IReadOnlyList<T>` or `IEnumerable<T>` for covariant read-only access.
7. **Accept the narrowest interface type your method needs.** Use `IEnumerable<T>` for iteration, `IReadOnlyList<T>` for indexed read access, and `IList<T>` only when mutation is required. This follows ISP and makes methods more reusable.
---
## Knowledge Sources
SOLID and DRY guidance in this skill is grounded in publicly available content from:
- **Steve Smith (Ardalis) SOLID Principles** -- Practical SOLID application in .NET with guard clause patterns, specification pattern for OCP compliance, and clean architecture layering that enforces DIP at project boundaries. Source: https://ardalis.com/
- **Jimmy Bogard's Domain-Driven Design Patterns** -- Rich domain model guidance that applies SRP to aggregate design (one aggregate root per bounded context) and OCP to domain event handling (new handlers without modifying existing ones). Note: MediatR is commercial for commercial use; apply the patterns with built-in mechanisms where possible. Source: https://www.jimmybogard.com/
> **Note:** This skill applies publicly documented guidance. It does not represent or speak for the named sources.
## References
- [SOLID Principles in C#](https://learn.microsoft.com/en-us/archive/msdn-magazine/2014/may/csharp-best-practices-dangers-of-violating-solid-principles-in-csharp)
- [Dependency Injection in .NET](https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection)
- [Covariance and Contravariance in Generics](https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance)
- [Clean Architecture (Ardalis)](https://github.com/ardalis/CleanArchitecture)
## Attribution
Adapted from [Aaronontheweb/dotnet-skills](https://github.com/Aaronontheweb/dotnet-skills) (MIT license).
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