DevFoundry/.devfoundry/rules/dotnet-csharp.md

Rules: ASP.NET / C#

1. Nullability & Types: Prefer explicit types and nullable reference type annotations where the project enables them.
2. Data Access: Use parameterized queries via SqlCommand, Dapper, or Entity Framework — never string-concatenated SQL.
3. Async: Use async/await for I/O-bound operations; avoid blocking calls like .Result or .Wait().
4. TDD: Write the xUnit/NUnit/MSTest test first, run it and confirm it fails for the right reason, then implement the minimum code to pass it. Do not write production code without a preceding failing test.
5. Dependency Injection: Register services via the DI container rather than static singletons, unless working within a legacy WebForms constraint.
6. Error Handling: Use structured exception handling and centralized logging; do not swallow exceptions silently.
7. Architecture: New or substantially modified web apps use ASP.NET Core MVC or Razor Pages with controllers, models, and views kept in separate files/folders — no business logic embedded in views, no data access called directly from a view or code-behind. WebForms/legacy exceptions must be justified in the spec.
8. Design by Contract: Public methods validate their preconditions (e.g., via guard clauses or ArgumentNullException/ArgumentException) at the top of the method and document postconditions in XML doc comments or the spec's Technical Design section; do not rely on callers to pass valid state unchecked.