/azure-functions

name: azure-functions
description: "Build, review, or migrate Azure Functions in .NET with correct execution model, isolated worker setup, bindings, DI, and Durable Functions patterns. USE FOR: working on Azure Functions in .NET; migrating from the in-process model to the isolated worker model; adding Durable Functions, bindings, or host configuration. DO NOT USE FOR: unrelated stacks; generic tasks that do not need this specific guidance. INVOKES: inspect the repository context, edit targeted files, and run relevant build, test, lint, or validation commands when changes are made."
compatibility: "Requires an Azure Functions project or a migration plan for one."

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

// Program.cs
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        services.AddApplicationInsightsTelemetryWorkerService();
        services.ConfigureFunctionsApplicationInsights();
        services.AddSingleton<IMyService, MyService>();
    })
    .Build();

host.Run();

HTTP Trigger Function

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

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

// 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

[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

[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 -