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
• Differences between in-process and isolated worker process
• Migrate C# app from in-process to isolated worker model
• Durable Functions overview
• Durable Functions best practices and diagnostic tools

References

• Patterns - Isolated worker patterns, Durable Functions patterns, advanced binding patterns
• Anti-Patterns - 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 - 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

[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 cts = new CancellationTokenSource();
    var approvalTask = context.WaitForExternalEvent<bool>("ApprovalEvent");
    var timeoutTask = context.CreateTimer(context.CurrentUtcDateTime.AddDays(7), cts.Token);

    var winner = await Task.WhenAny(approvalTask, timeoutTask);

    if (winner == approvalTask)
    {
        cts.Cancel();
        return approvalTask.Result ? "Approved" : "Rejected";
    }

    return "Timed out";
}

Best Practices

1. Use isolated worker model for new development - Full .NET ecosystem access, middleware support, and longer support lifecycle
2. Inject dependencies via constructor - Use ILogger<T> and service interfaces for testability
3. Keep orchestrator code deterministic - No I/O, random, DateTime.Now, or Guid.NewGuid() in orchestrators
4. Handle sensitive data in activities - Fetch secrets from Key Vault in activity functions, never in orchestrators
5. Use unique task hub names - Prevent accidental sharing when multiple apps use the same storage
6. Avoid large inputs/outputs - Serialize to blob storage for large payloads to prevent history bloat
7. Configure concurrency limits - Set appropriate limits in host.json for resource-intensive functions
8. Keep SDK and extensions updated - Latest versions include performance improvements and bug fixes

Anti-Patterns to Avoid

Anti-Pattern	Why It's Bad	Better Approach
Mixing in-process and isolated guidance	Incompatible APIs and patterns	Choose one model consistently
Non-deterministic orchestrator code	Replay failures, stuck orchestrations	Use context.CurrentUtcDateTime, no I/O
Large orchestrator inputs/outputs	History bloat, memory issues	Store large data in blob storage
Shared task hub names	Message conflicts, stuck orchestrations	Use unique names per app
Secrets in orchestrator history	Security risk, exposed in logs	Fetch secrets in activity functions
Blocking calls in async functions	Thread pool exhaustion	Use await throughout
Missing retry policies	Transient failures cause job loss	Configure retry in bindings or code
Ignoring execution model migration	EOL November 2026 for in-process	Migrate to isolated worker model

Deployment Considerations

Linux Consumption Plan Limitations

.NET 10+ apps cannot run on Linux Consumption plan.
Use Flex Consumption plan or App Service for .NET 10+.
.NET 9 is the last version supported on Linux Consumption.

host.json Configuration

{
  "version": "2.0",
  "extensions": {
    "durableTask": {
      "hubName": "MyUniqueTaskHub",
      "maxConcurrentActivityFunctions": 10,
      "maxConcurrentOrchestratorFunctions": 5
    }
  },
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      }
    }
  }
}

Deliver

• correct Functions project setup for the isolated worker model
• clear binding and host configuration
• middleware for cross-cutting concerns
• Durable Functions with proper orchestration patterns
• migration-safe guidance when upgrading execution models

Validate

• execution model guidance is consistent (isolated only for new work)
• orchestrator code is deterministic
• bindings and host settings match the target runtime
• large payloads are externalized to blob storage
• retry policies are configured for transient failures
• local and deployment behavior are both verified