Azure-Functions-Csharp

# Azure Functions C# Development

General Instructions

- Always use the **isolated worker model** (not the legacy in-process model) for all new Azure Functions projects targeting .NET 6 or later.

- Use `FunctionsApplication.CreateBuilder(args)` or `HostBuilder` in `Program.cs` for host setup and dependency injection.

- Decorate function methods with `[Function("FunctionName")]` and use strongly typed trigger and binding attributes.

- Keep function methods focused — each function should do one thing and delegate business logic to injected services.

- Never put business logic directly inside the function method body; extract it into testable service classes registered via DI.

- Use `ILogger<T>` injected through the constructor, not `ILogger` passed as a function parameter, for consistent structured logging.

- Always use `async/await` for all I/O-bound operations; never block with `.Result` or `.Wait()`.

- Prefer `CancellationToken` parameters where supported to enable graceful shutdown.

Project Structure and Setup

- Use the `Microsoft.Azure.Functions.Worker` and `Microsoft.Azure.Functions.Worker.Extensions.*` NuGet packages.

- Register services in `Program.cs` using `builder.Services.Add*` extension methods for clean dependency injection.

- Group related functions into separate classes by domain concern, not by trigger type.

- Store configuration in `local.settings.json` for local development; use Azure App Configuration or Application Settings for deployed environments.

- Never hardcode connection strings or secrets in code; always read from `IConfiguration` or environment variables.

- Use Key Vault references (`@Microsoft.KeyVault(SecretUri=...)`) in App Settings for secrets in deployed environments.

- Use `Managed Identity` (`DefaultAzureCredential`) for authenticating to Azure services — avoid connection strings with keys wherever possible.

- Keep `host.json` tuned per trigger type: configure `maxConcurrentCalls`, `batchSize`, and retry policies at the host level.

Triggers

- **HttpTrigger**: Use `AuthorizationLevel.Function` or higher for production endpoints; reserve `AuthorizationLevel.Anonymous` only for public-facing APIs with explicit justification. Use ASP.NET Core integration (`UseMiddleware`, `IActionResult` returns) when using the ASP.NET Core integration model.

- **TimerTrigger**: Use NCRONTAB expressions (`"0 */5 * * * *"`) for schedules; avoid `RunOnStartup = true` in production as it executes immediately on every cold start.

- **QueueTrigger / ServiceBusTrigger**: Configure `MaxConcurrentCalls`, dead-letter policies, and `MaxDeliveryCount` in `host.json` and Azure portal; handle `ServiceBusReceivedMessage` directly for advanced message control (complete, abandon, dead-letter).

- **BlobTrigger**: Prefer Event Grid-based blob triggers (`Microsoft.Azure.Functions.Worker.Extensions.EventGrid`) over polling-based blob triggers for lower latency and reduced storage transaction costs.

- **EventHubTrigger**: Set `cardinality` to `many` for batch processing; use `EventData[]` or `string[]` parameter types for batch mode; always checkpoint using the `EventHubTriggerAttribute`'s built-in checkpointing.

- **CosmosDBTrigger**: Use the change feed trigger for event-driven processing of Cosmos DB changes; set `LeaseContainerName` and manage lease containers separately from data containers.

Input and Output Bindings

- Use input bindings to read data declaratively rather than using SDKs directly inside function bodies where the binding covers the use case.

- For multiple output bindings, define a custom return type with properties annotated with the appropriate output binding attributes (e.g., `[QueueOutput]`, `[BlobOutput]`, `[HttpResult]`).

- Use `[BlobInput]` and `[BlobOutput]` for blob read/write; prefer `Stream` over `byte[]` for large blobs to avoid memory pressure.

- Use `[CosmosDBInput]` for point reads and simple queries; for complex queries, inject `CosmosClient` via DI with `Managed Identity`.

- Use