Migrating Azure Functions Startup

Azure Functions Startup Hooks Migration

Overview

Migrate Azure Functions from the in-process model (using FunctionsStartup and IFunctionsHostBuilder) to the isolated worker model, moving all service registrations and configuration from Startup.cs into Program.cs.

Related skill: For upgrading an already-isolated project to the V2 hosting pattern (IHostApplicationBuilder), use migrating-azure-functions-to-v2 instead.

The startup file may not be named Startup.cs.Identify it by looking for classes inheriting FunctionsStartup, usage of IFunctionsHostBuilder or FunctionsHostBuilderContext, or the [assembly: FunctionsStartup(...)] attribute. Throughout this skill, "Startup.cs" refers to whichever file contains these patterns.

Workflow

Track progress using this checklist:

Migration Progress:
- [ ] Phase 1, Step 1: Get target framework
- [ ] Phase 1, Step 2: Determine in-process vs isolated
- [ ] Phase 1, Step 3: Determine upgrade path
- [ ] Phase 1, Step 4: Check ASP.NET Core integration
- [ ] Phase 2, Step 1: Remove obsolete packages
- [ ] Phase 2, Step 2: Comment out Startup.cs
- [ ] Phase 2, Step 3: Create or update Program.cs
- [ ] Phase 2, Step 4: Migrate service registrations
- [ ] Phase 2, Step 5: Handle build failures
- [ ] Phase 2, Step 6: Verify

Phase 1: Assessment

Step 1: Get Target Framework

Open the project file and note <TargetFramework> (net6.0, net8.0, net9.0, etc.).

Step 2: Determine In-Process vs Isolated

In-process indicators (project file):

<PackageReference Include="Microsoft.NET.Sdk.Functions" Version="4.x.x" />
<PackageReference Include="Microsoft.Azure.WebJobs.Extensions.Http" Version="3.x.x" />

Isolated indicators (project file):

<PackageReference Include="Microsoft.Azure.Functions.Worker" Version="x.x.x" />
<PackageReference Include="Microsoft.Azure.Functions.Worker.Sdk" Version="x.x.x" />

Additional signals: in-process uses [FunctionName] attribute; isolated has Program.cs with a host builder.

Step 3: Determine Upgrade Path

• Already isolated → skip to Phase 2.
• .NET 8 in-process → ask user whether to also upgrade the framework, or just switch to isolated model.
• .NET 6 in-process → upgrade to .NET 8+ using isolated worker model (start .NET upgrade scenario flow).

Step 4: Check ASP.NET Core Integration

Determine whether the project uses ASP.NET Core integration. This controls the builder method in Program.cs:

• With ASP.NET Core → ConfigureFunctionsWebApplication()
• Without → ConfigureFunctionsWorkerDefaults()

Phase 2: Execution

Step 1: Remove Obsolete Packages

Remove Microsoft.Azure.Functions.Extensions from the project file. This package only supports in-process and causes build conflicts with the isolated model.

Step 2: Comment Out Startup.cs

Comment out the entire class rather than deleting the file — this preserves the original code as a rollback reference until the user verifies the migration works.

// Logic has been moved to Program.cs. Delete this file once migration is verified.
/*
[assembly: FunctionsStartup(typeof(MyNamespace.Startup))]

namespace MyNamespace
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            builder.Services.AddSingleton<IMyService, MyService>();
        }
    }
}
*/

Step 3: Create or Update Program.cs

Use the appropriate builder method based on Step 4 above.

Without ASP.NET Core integration:

using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

var host = new HostBuilder()
    .ConfigureFunctionsWorkerDefaults()
    .ConfigureServices((context, services) =>
    {
        // Migrated from Startup.cs
    })
    .Build();

host.Run();

With ASP.NET Core integration:

using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices((context, services) =>
    {
        // Migrated from Startup.cs
    })
    .Build();

host.Run();

Step 4: Migrate Service Registrations

Move all registrations from IFunctionsHostBuilder.Services into the ConfigureServices lambda.

If FunctionsHostBuilderContext was used, replace with HostBuilderContext and add a comment explaining the scope change — in the in-process model, custom configuration could influence the host; in isolated mode it applies to the worker only.

Before (Startup.cs):

public override void Configure(IFunctionsHostBuilder builder)
{
    var context = builder.GetContext();
    var connectionString = context.Configuration["ConnectionStrings:MyDb"];

    builder.Services.AddSingleton<IMyService, MyService>();
    builder.Services.AddDbContext<MyDbContext>(options =>
        options.UseSqlServer(connectionString));
}

After (Program.cs):

.ConfigureServices((context, services) =>
{
    // Note: Custom configuration sources now apply to worker only (not host).
    var connectionString = context.Configuration["ConnectionStrings:MyDb"];

    services.AddSingleton<IMyService, MyService>();
    services.AddDbContext<MyDbContext>(options =>
        options.UseSqlServer(connectionString));
})

Step 5: Handle Build Failures

If build errors persist after migration, add a placeholder to signal incomplete migration and notify the user that manual review is required:

.ConfigureServices((context, services) =>
{
    throw new NotImplementedException(
        "Migrate services from Startup.cs to ConfigureServices within Program.cs.");
})

Step 6: Verify

Confirm:

• Isolated worker packages are referenced; no in-process packages remain
• Microsoft.Azure.Functions.Extensions is removed
• Startup.cs is commented out with a deletion note
• Program.cs uses the correct builder method
• All service registrations and configuration are migrated
• Project builds without errors

Notify the user to review the migration, test all functions, and delete Startup.cs once verified.

Success Criteria

• Project uses isolated worker model (Microsoft.Azure.Functions.Worker packages)
• No in-process packages remain (Microsoft.NET.Sdk.Functions, Microsoft.Azure.WebJobs.*)
• Microsoft.Azure.Functions.Extensions removed
• Startup.cs commented out (not deleted) with migration note
• Program.cs uses correct builder method
• All service registrations migrated; FunctionsHostBuilderContext replaced with HostBuilderContext
• Project builds without errors

References

• Migrate to isolated worker model
• Isolated process guide
• Dependency injection in Azure Functions
• ASP.NET Core integration