.NET/C# Best Practices

C# coding guidelines and best practices. MUST follow these rules. Use when reviewing/writing C# code or dotnet tasks.

Documentation & Structures

• Create comprehensive XML documentation comments for all public classes, interfaces, methods, and properties using the csharp-docs skill.
• Include parameter descriptions and return value descriptions in XML comments
• Follow the established namespace structure: {Core|Console|App|Service}.{Feature}

Design Patterns & Architecture

• Use primary constructor syntax for dependency injection (e.g., public class MyClass(IDependency dependency))
• Implement the Command Handler pattern with generic base classes (e.g., CommandHandler<TOptions>)
• Use interface segregation with clear naming conventions (prefix interfaces with 'I')
• Follow the Factory pattern for complex object creation.

Syntax & Language Features

• Prefer Records for DTOs and messages to ensure immutability by default.
• NEVER mix multiple classes or DTOs in a single file, even if they are small. Filename ALWAYS matches the class or DTO name, for example UserDto.cs for a UserDto class.
• Use record structs and init accessors so that you can clone objects using the with keyword rather than tracking state mutations.
• Leverage Pattern Matching with switch expressions for cleaner logic flow.
• Utilize Collection Expressions and Span for high-performance data handling.
• Use expression bodies (arrow syntax) for methods, properties, and constructors when the logic is a simple transformation.
• For conditional assignments, replace bulky, nested if-else blocks or legacy switch statements with switch expressions.
• Use [ and ] for initializing arrays, lists, and spans. This offers a uniform, clean syntax across all collection types and eliminates redundant type declarations.
• One Class Per File: Ensure a strict structure with matching filenames.
• Use var for local variables when type is obvious.
• ALWAYS use sealed accessor for classes or records if not intended for inheritance.
• ALWAYS use internal accessor for internal classes or records.
• Don't use Regions in very small code files.
• Use Regions in large code files to group public properties.
• Use Regions in large code files to group event logic together.

Dependency Injection & Services

• Use constructor dependency injection with null checks via ArgumentNullException
• Register services with appropriate lifetimes (Singleton, Scoped, Transient)
• Use Microsoft.Extensions.DependencyInjection patterns
• Implement service interfaces for testability

Resource Management & Localization

• Use ResourceManager for localized messages and error strings
• Separate LogMessages and ErrorMessages resource files
• Access resources via _resourceManager.GetString("MessageKey")

Async/Await Patterns

• Use async/await for all I/O operations and long-running tasks
• Return Task or Task from async methods
• Use ConfigureAwait(false) where appropriate
• Handle async exceptions properly

Testing Standards

• Use XUnit framework for unit tests.
• Follow AAA pattern (Arrange, Act, Assert)
• Use Moq for mocking dependencies
• Test both success and failure scenarios
• Include null parameter validation tests

Configuration & Settings

• Use strongly-typed configuration classes with data annotations
• Implement validation attributes (Required, NotEmptyOrWhitespace)
• Use IConfiguration binding for settings
• Support appsettings.json configuration files

Semantic Kernel & AI Integration

• Use Microsoft.SemanticKernel for AI operations
• Implement proper kernel configuration and service registration
• Handle AI model settings (ChatCompletion, Embedding, etc.)
• Use structured output patterns for reliable AI responses

Error Handling & Logging

• Use structured logging with Microsoft.Extensions.Logging or NLog
• Prefer NLog if it is already used
• ALWAYS Structured logging to log, no string interpolation and no other logging methods.
• Include scoped logging with meaningful context
• Throw specific exceptions with descriptive messages
• Use try-catch blocks for expected failure scenarios
• Use early exists with ArgumentNullException.ThrowIfNull(argument) instead of custom if (argument == null) checks.

Performance & Security

• Use C# 12+ features and .NET 8+ optimizations where applicable
• Implement proper input validation and sanitization
• Use parameterized queries for database operations
• Follow secure coding practices for AI/ML operations

Code Quality

• Ensure SOLID principles compliance
• Avoid code duplication through base classes and utilities
• Use meaningful names that reflect domain concepts
• Keep methods focused and cohesive
• Implement proper disposal patterns for resources

WPF

• UI Agnosticism: Never use WPF types (e.g., MessageBox, Panel, TextBox) inside the ViewModel. All UI interactions must go through interfaces or services (e.g., IDialogService).
• No Code-Behind: Only use XAML and Microsoft.Xaml.Behaviors.Wpf (EventToCommand) to trigger ViewModel logic. Code-behind must be strictly reserved for View-specific visual setups.
• One-to-One Binding: Enforce one View-Model per View and rely purely on data binding (e.g., {Binding PropertyName}) rather than accessing UI elements by their x:Name

Winforms

• Do not create any new UI using Winforms.
• Legacy Winforms UI can be maintained.