azure-functions

# Azure Functions for .NET

## Trigger On

- working on Azure Functions in .NET
- migrating from the in-process model to the isolated worker model
- adding Durable Functions, bindings, or host configuration

## Documentation

- [Guide for running C# Azure Functions in an isolated worker process](https://learn.microsoft.com/en-us/azure/azure-functions/dotnet-isolated-process-guide)
- [Differences between in-process and isolated worker process](https://learn.microsoft.com/en-us/azure/azure-functions/dotnet-isolated-in-process-differences)
- [Migrate C# app from in-process to isolated worker model](https://learn.microsoft.com/en-us/azure/azure-functions/migrate-dotnet-to-isolated-model)
- [Durable Functions overview](https://learn.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-overview)
- [Durable Functions best practices and diagnostic tools](https://learn.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-best-practice-reference)

### References

- [Patterns](references/patterns.md) - Isolated worker patterns, Durable Functions patterns, advanced binding patterns
- [Anti-Patterns](references/anti-patterns.md) - Common Azure Functions mistakes and how to avoid them

## Workflow

1. **Use isolated worker model for all new work:**
   - In-process model reaches end of support on November 10, 2026
   - Runtime v1.x ends support on September 14, 2026
   - Target .NET 8+ for longest support window

2. **Detect current project shape:**
   - Target framework and runtime version
   - Worker model (isolated vs in-process)
   - Binding packages and host configuration

3. **Use standard .NET patterns in isolated model:**
   - Normal dependency injection
   - Middleware pipeline
   - `IOptions<T>` for configuration
   - `ILogger<T>` for logging

4. **For Durable Functions:**
   - Validate orchestration determinism constraints
   - Handle replay behavior correctly
   - Use typed activity patterns

5. **Verify both local and deployment behavior.**

## Isolated Worker Model Setup

### Basic Function with DI
```csharp
// Program.cs
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        services.AddApplicationInsightsTelemetryWorkerService();
        services.ConfigureFunctionsApplicationInsights();
        services.AddSingleton<IMyService, MyService>();
    })
    .Build();

host.Run();
```

### HTTP Trigger Function
```csharp
public class HttpFunctions(ILogger<HttpFunctions> logger, IMyService myService)
{
    [Function("GetItems")]
    public async Task<IActionResult> GetItems(
        [HttpTrigger(AuthorizationLevel.Function, "get", Route = "items")] HttpRequest req)
    {
        logger.LogInformation("Processing GetItems request");
        var items = await myService.GetItemsAsync();
        return new OkObjectResult(items);
    }
}
```

### Queue Trigger with Options
```csharp
public class QueueFunctions(ILogger<QueueFunctions> logger, IOptions<ProcessingOptions> options)
{
    [Function("ProcessMessage")]
    public async Task ProcessMessage(
        [QueueTrigger("myqueue", Connection = "AzureWebJobsStorage")] string message)
    {
        logger.LogInformation("Processing message: {Message}", message);
        // Process with retry policy from options
    }
}
```

## Middleware Pattern

### Custom Middleware
```csharp
// Program.cs
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication(builder =>
    {
        builder.UseMiddleware<ExceptionHandlingMiddleware>();
        builder.UseMiddleware<CorrelationIdMiddleware>();
    })
    .Build();

// CorrelationIdMiddleware.cs
public class CorrelationIdMiddleware : IFunctionsWorkerMiddleware
{
    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        var correlationId = context.Features.Get<IHttpRequestFeature>()?.Headers["X-Correlation-Id"]
            ?? Guid.NewGuid().ToString();

        context.Items["CorrelationId"] = correlationId;

        await next(context);
    }
}
```

## Durable Functions Patterns

### Function Chaining
```csharp
[Function(nameof(ChainOrchestrator))]
public static async Task<string> ChainOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var result1 = await context.CallActivityAsync<string>(nameof(Step1), "input");
    var result2 = await context.CallActivityAsync<string>(nameof(Step2), result1);
    var result3 = await context.CallActivityAsync<string>(nameof(Step3), result2);
    return result3;
}

[Function(nameof(Step1))]
public static string Step1([ActivityTrigger] string input) => $"Step1({input})";

[Function(nameof(Step2))]
public static string Step2([ActivityTrigger] string input) => $"Step2({input})";

[Function(nameof(Step3))]
public static string Step3([ActivityTrigger] string input) => $"Step3({input})";
```

### Fan-Out/Fan-In
```csharp
[Function(nameof(FanOutFanInOrchestrator))]
public static async Task<int[]> FanOutFanInOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var workItems = await context.CallActivityAsync<string[]>(nameof(GetWorkItems), null);

    // Fan out - process all items in parallel
    var tasks = workItems.Select(item =>
        context.CallActivityAsync<int>(nameof(ProcessWorkItem), item));

    // Fan in - wait for all to complete
    var results = await Task.WhenAll(tasks);

    return results;
}

[Function(nameof(ProcessWorkItem))]
public static int ProcessWorkItem([ActivityTrigger] string item)
{
    // Process item and return result
    return item.Length;
}
```

### Human Interaction Pattern
```csharp
[Function(nameof(ApprovalOrchestrator))]
public static async Task<string> ApprovalOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var request = context.GetInput<ApprovalRequest>();

    // Send notification
    await context.CallActivityAsync(nameof(SendApprovalRequest), request);

    // Wait for external event with timeout
    using var