Azure Functions

Overview

Azure Functions is a serverless compute platform that lets you run event-driven code without managing infrastructure. The .NET isolated worker model (recommended for .NET 8+) runs functions in a separate process with full control over the host, middleware pipeline, and dependency injection. Functions can be triggered by HTTP requests, timers, queues, blobs, Event Hubs, and more.

NuGet Packages

bash

dotnet add package Microsoft.Azure.Functions.Worker
dotnet add package Microsoft.Azure.Functions.Worker.Sdk
dotnet add package Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore
dotnet add package Microsoft.Azure.Functions.Worker.Extensions.Timer
dotnet add package Microsoft.Azure.Functions.Worker.Extensions.Storage.Queues
dotnet add package Microsoft.Azure.Functions.Worker.Extensions.Storage.Blobs
dotnet add package Microsoft.Azure.Functions.Worker.Extensions.DurableTask

Project Setup (Isolated Worker Model)

csharp

// Program.cs
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        services.AddApplicationInsightsTelemetryWorkerService();
        services.ConfigureFunctionsApplicationInsights();

        // Register your services
        services.AddHttpClient();
        services.AddSingleton<IOrderService, OrderService>();
        services.AddDbContext<AppDbContext>(options =>
            options.UseSqlServer(
                Environment.GetEnvironmentVariable("SqlConnectionString")));
    })
    .Build();

host.Run();

HTTP Trigger

csharp

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;

public class OrderFunctions(IOrderService orderService, ILogger<OrderFunctions> logger)
{
    [Function("GetOrders")]
    public async Task<IActionResult> GetOrders(
        [HttpTrigger(AuthorizationLevel.Function, "get", Route = "orders")] HttpRequest req)
    {
        logger.LogInformation("Getting all orders");
        var orders = await orderService.GetAllAsync();
        return new OkObjectResult(orders);
    }

    [Function("GetOrderById")]
    public async Task<IActionResult> GetOrderById(
        [HttpTrigger(AuthorizationLevel.Function, "get", Route = "orders/{id:int}")] HttpRequest req,
        int id)
    {
        var order = await orderService.GetByIdAsync(id);
        return order is null
            ? new NotFoundResult()
            : new OkObjectResult(order);
    }

    [Function("CreateOrder")]
    public async Task<IActionResult> CreateOrder(
        [HttpTrigger(AuthorizationLevel.Function, "post", Route = "orders")] HttpRequest req)
    {
        var order = await req.ReadFromJsonAsync<CreateOrderRequest>();
        if (order is null)
            return new BadRequestObjectResult("Invalid order data");

        var created = await orderService.CreateAsync(order);
        return new CreatedResult($"/api/orders/{created.Id}", created);
    }
}

Timer Trigger

csharp

public class ScheduledFunctions(ILogger<ScheduledFunctions> logger)
{
    // Runs every 5 minutes
    [Function("CleanupExpiredSessions")]
    public async Task CleanupExpiredSessions(
        [TimerTrigger("0 */5 * * * *")] TimerInfo timer)
    {
        logger.LogInformation("Cleanup started at: {Time}", DateTime.UtcNow);

        if (timer.ScheduleStatus is not null)
        {
            logger.LogInformation("Next occurrence: {Next}", timer.ScheduleStatus.Next);
        }

        // Cleanup logic here
        await Task.CompletedTask;
    }

    // Runs daily at 2:00 AM UTC
    [Function("DailyReport")]
    public async Task GenerateDailyReport(
        [TimerTrigger("0 0 2 * * *")] TimerInfo timer)
    {
        logger.LogInformation("Generating daily report");
        await Task.CompletedTask;
    }
}

Queue Trigger and Output

csharp

public class QueueFunctions(IOrderService orderService, ILogger<QueueFunctions> logger)
{
    [Function("ProcessOrder")]
    [QueueOutput("order-notifications")]
    public async Task<string> ProcessOrder(
        [QueueTrigger("incoming-orders")] string orderJson)
    {
        var order = JsonSerializer.Deserialize<Order>(orderJson)!;
        logger.LogInformation("Processing order: {OrderId}", order.Id);

        await orderService.ProcessAsync(order);

        // Return value is sent to the output queue
        return JsonSerializer.Serialize(new
        {
            orderId = order.Id,
            status = "processed",
            timestamp = DateTime.UtcNow
        });
    }
}

// Multiple outputs
public class MultiOutputFunction
{
    [Function("ProcessAndNotify")]
    public async Task<MultiOutput> ProcessAndNotify(
        [QueueTrigger("work-items")] WorkItem item)
    {
        return new MultiOutput
        {
            CompletedItem = JsonSerializer.Serialize(new { item.Id, Status = "done" }),
            NotificationMessage = $"Work item {item.Id} completed"
        };
    }
}

public class MultiOutput
{
    [QueueOutput("completed-items")]
    public string CompletedItem { get; set; } = "";

    [QueueOutput("notifications")]
    public string NotificationMessage { get; set; } = "";
}

Blob Trigger

csharp

public class BlobFunctions(ILogger<BlobFunctions> logger)
{
    [Function("ProcessUploadedImage")]
    [BlobOutput("thumbnails/{name}", Connection = "StorageConnection")]
    public async Task<byte[]> ProcessUploadedImage(
        [BlobTrigger("uploads/{name}", Connection = "StorageConnection")] Stream inputBlob,
        string name)
    {
        logger.LogInformation("Processing blob: {Name}, Size: {Size}", name, inputBlob.Length);

        using var image = await Image.LoadAsync(inputBlob);
        image.Mutate(x => x.Resize(150, 150));

        using var outputStream = new MemoryStream();
        await image.SaveAsJpegAsync(outputStream);
        return outputStream.ToArray();
    }
}

Trigger Types

Trigger	Package	Cron/Schedule
HTTP	`Extensions.Http.AspNetCore`	On-demand
Timer	`Extensions.Timer`	`0 /5 * * *` (every 5 min)
Queue	`Extensions.Storage.Queues`	Message arrival
Blob	`Extensions.Storage.Blobs`	Blob creation/update
Event Hub	`Extensions.EventHubs`	Event arrival
Service Bus	`Extensions.ServiceBus`	Message arrival
Cosmos DB	`Extensions.CosmosDB`	Document change

Durable Functions (Orchestration)

csharp

using Microsoft.Azure.Functions.Worker;
using Microsoft.DurableTask;
using Microsoft.DurableTask.Client;

public class OrderOrchestration
{
    [Function("OrderWorkflow")]
    public static async Task<OrderResult> RunOrchestrator(
        [OrchestrationTrigger] TaskOrchestrationContext context)
    {
        var order = context.GetInput<Order>()!;

        // Step 1: Validate inventory
        var isAvailable = await context.CallActivityAsync<bool>(
            "CheckInventory", order);

        if (!isAvailable)
            return new OrderResult(order.Id, "Failed", "Out of stock");

        // Step 2: Process payment
        var paymentResult = await context.CallActivityAsync<PaymentResult>(
            "ProcessPayment", order);

        if (!paymentResult.Success)
            return new OrderResult(order.Id, "Failed", paymentResult.Error);

        // Step 3: Ship order
        var trackingNumber = await context.CallActivityAsync<string>(
            "ShipOrder", order);

        // Step 4: Send confirmation
        await context.CallActivityAsync("SendConfirmation", new
        {
            order.Id,
            order.CustomerEmail,
            trackingNumber
        });

        return new OrderResult(order.Id, "Completed", trackingNumber);
    }

    [Function("CheckInventory")]
    public static async Task<bool> CheckInventory(
        [ActivityTrigger] Order order, ILogger logger)
    {
        logger.LogInformation("Checking inventory for order {OrderId}", order.Id);
        // Check inventory logic
        return true;
    }

    [Function("ProcessPayment")]
    public static async Task<PaymentResult> ProcessPayment(
        [ActivityTrigger] Order order)
    {
        // Payment processing logic
        return new PaymentResult(true, null);
    }

    [Function("ShipOrder")]
    public static async Task<string> ShipOrder(
        [ActivityTrigger] Order order)
    {
        // Shipping logic
        return $"TRACK-{Guid.NewGuid():N}"[..16];
    }

    [Function("SendConfirmation")]
    public static Task SendConfirmation(
        [ActivityTrigger] dynamic details, ILogger logger)
    {
        logger.LogInformation("Sending confirmation for order {OrderId}", (string)details.Id);
        return Task.CompletedTask;
    }

    // HTTP trigger to start the orchestration
    [Function("StartOrderWorkflow")]
    public static async Task<IActionResult> StartOrderWorkflow(
        [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req,
        [DurableClient] DurableTaskClient client)
    {
        var order = await req.ReadFromJsonAsync<Order>();
        var instanceId = await client.ScheduleNewOrchestrationInstanceAsync(
            "OrderWorkflow", order);

        return new AcceptedResult($"/api/status/{instanceId}", new { instanceId });
    }
}

record OrderResult(int Id, string Status, string? Details);
record PaymentResult(bool Success, string? Error);

Middleware

csharp

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication(worker =>
    {
        worker.UseMiddleware<ExceptionHandlingMiddleware>();
        worker.UseMiddleware<CorrelationIdMiddleware>();
    })
    .Build();

public class ExceptionHandlingMiddleware(ILogger<ExceptionHandlingMiddleware> logger)
    : IFunctionsWorkerMiddleware
{
    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        try
        {
            await next(context);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, "Unhandled exception in {Function}", context.FunctionDefinition.Name);
            throw;
        }
    }
}

local.settings.json

json

{
  "IsEncrypted": false,
  "Values": {
    "AzureWebJobsStorage": "UseDevelopmentStorage=true",
    "FUNCTIONS_WORKER_RUNTIME": "dotnet-isolated",
    "SqlConnectionString": "Server=localhost;Database=myapp;Trusted_Connection=true;"
  }
}

Best Practices

•Use the isolated worker model (ConfigureFunctionsWebApplication) for all new .NET 8+ projects; the in-process model is deprecated and does not support .NET 8+.
•Register services in ConfigureServices and use constructor injection in function classes rather than creating instances manually; function classes support the same DI patterns as ASP.NET Core controllers.
•Use AuthorizationLevel.Function for API endpoints that need key-based access control, and AuthorizationLevel.Anonymous only for public endpoints or when using a reverse proxy (API Management, Front Door) that handles auth.
•Set Route = "orders/{id:int}" on HTTP triggers with route constraints to get type-safe parameter binding and clear URL patterns rather than parsing query strings manually.
•Use Durable Functions (OrchestrationTrigger + ActivityTrigger) for multi-step workflows that need reliable execution, rather than chaining queue messages between separate functions.
•Keep function execution time under 5 minutes on the Consumption plan; use the Premium or Dedicated plan for longer-running functions, and Durable Functions for workflows that span hours or days.
•Store connection strings and secrets in local.settings.json for local development and in Azure Application Settings (or Key Vault references) for production; never commit local.settings.json to source control.
•Use [QueueOutput] and [BlobOutput] attributes for output bindings rather than creating SDK clients manually, which reduces boilerplate and leverages the runtime's connection management.
•Add Application Insights with AddApplicationInsightsTelemetryWorkerService() to get automatic request tracking, dependency tracking, and exception logging without manual instrumentation.
•Test functions by creating instances of your function class directly and passing mock services through constructor injection; use HttpRequestData or test doubles for trigger inputs.