GHCP-csharp-instructions.md

description	applyTo
Guidelines for building C# applications	**/*.cs

C# Development

C# Instructions

• Always use the latest version C#, currently C# 13 features.
• Write clear and concise comments for each function.

General Instructions

• Make only high confidence suggestions when reviewing code changes.
• Write code with good maintainability practices, including comments on why certain design decisions were made.
• Handle edge cases and write clear exception handling.
• For libraries or external dependencies, mention their usage and purpose in comments.

Naming Conventions

• Follow PascalCase for component names, method names, and public members.
• Use camelCase for private fields and local variables.
• Prefix interface names with "I" (e.g., IUserService).

Formatting

• Apply code-formatting style defined in .editorconfig.
• Prefer file-scoped namespace declarations and single-line using directives.
• Insert a newline before the opening curly brace of any code block (e.g., after if, for, while, foreach, using, try, etc.).
• Ensure that the final return statement of a method is on its own line.
• Use pattern matching and switch expressions wherever possible.
• Use nameof instead of string literals when referring to member names.
• Ensure that XML doc comments are created for any public APIs. When applicable, include <example> and <code> documentation in the comments.

Project Setup and Structure

• Guide users through creating a new .NET project with the appropriate templates.
• Explain the purpose of each generated file and folder to build understanding of the project structure.
• Demonstrate how to organize code using feature folders or domain-driven design principles.
• Show proper separation of concerns with models, services, and data access layers.
• Explain the Program.cs and configuration system in ASP.NET Core 9 including environment-specific settings.

Nullable Reference Types

• Declare variables non-nullable, and check for null at entry points.
• Always use is null or is not null instead of == null or != null.
• Trust the C# null annotations and don't add null checks when the type system says a value cannot be null.

Data Access Patterns

• Guide the implementation of a data access layer using Entity Framework Core.
• Explain different options (SQL Server, SQLite, In-Memory) for development and production.
• Demonstrate repository pattern implementation and when it's beneficial.
• Show how to implement database migrations and data seeding.
• Explain efficient query patterns to avoid common performance issues.

Authentication and Authorization

• Guide users through implementing authentication using JWT Bearer tokens.
• Explain OAuth 2.0 and OpenID Connect concepts as they relate to ASP.NET Core.
• Show how to implement role-based and policy-based authorization.
• Demonstrate integration with Microsoft Entra ID (formerly Azure AD).
• Explain how to secure both controller-based and Minimal APIs consistently.

Validation and Error Handling

• Guide the implementation of model validation using data annotations and FluentValidation.
• Explain the validation pipeline and how to customize validation responses.
• Demonstrate a global exception handling strategy using middleware.
• Show how to create consistent error responses across the API.
• Explain problem details (RFC 7807) implementation for standardized error responses.

API Versioning and Documentation

• Guide users through implementing and explaining API versioning strategies.
• Demonstrate Swagger/OpenAPI implementation with proper documentation.
• Show how to document endpoints, parameters, responses, and authentication.
• Explain versioning in both controller-based and Minimal APIs.
• Guide users on creating meaningful API documentation that helps consumers.

Logging and Monitoring

• Guide the implementation of structured logging using Serilog or other providers.
• Explain the logging levels and when to use each.
• Demonstrate integration with Application Insights for telemetry collection.
• Show how to implement custom telemetry and correlation IDs for request tracking.
• Explain how to monitor API performance, errors, and usage patterns.

Testing

• Always include test cases for critical paths of the application.
• Guide users through creating unit tests.
• Do not emit "Act", "Arrange" or "Assert" comments.
• Copy existing style in nearby files for test method names and capitalization.
• Explain integration testing approaches for API endpoints.
• Demonstrate how to mock dependencies for effective testing.
• Show how to test authentication and authorization logic.
• Explain test-driven development principles as applied to API development.

Performance Optimization

• Guide users on implementing caching strategies (in-memory, distributed, response caching).
• Explain asynchronous programming patterns and why they matter for API performance.
• Demonstrate pagination, filtering, and sorting for large data sets.
• Show how to implement compression and other performance optimizations.
• Explain how to measure and benchmark API performance.

Deployment and DevOps

• Guide users through containerizing their API using .NET's built-in container support (dotnet publish --os linux --arch x64 -p:PublishProfile=DefaultContainer).
• Explain the differences between manual Dockerfile creation and .NET's container publishing features.
• Explain CI/CD pipelines for NET applications.
• Demonstrate deployment to Azure App Service, Azure Container Apps, or other hosting options.
• Show how to implement health checks and readiness probes.
• Explain environment-specific configurations for different deployment stages.