With the release of .NET 9, developers are eager to adopt its powerful new features—NativeAOT, Blazor United, and improved Minimal APIs. But one question remains: How do you safely upgrade your existing .NET app to .NET 9?

This guide walks you through everything you need to know about migrating to .NET 9—whether you're coming from .NET 6, 7, or 8. We’ll cover compatibility checks, tool upgrades, code changes, testing, and deployment tips.

🛠 Prerequisites Before Migration

Before you jump into upgrading, ensure the following:

✅ Backup Your Project

Always create a full backup or use source control (Git) to avoid any accidental data loss during migration.

✅ Install .NET 9 SDK

Download the latest SDK from official .NET site. Make sure you're using a compatible version of Visual Studio (e.g., VS 2025 Preview or newer).

✅ Understand Support Lifecycle

.NET 9 is an STS (Standard Term Support) version. It gets 18 months of support, unlike LTS releases (e.g., .NET 8 with 3 years of support).

🔍 Step 1: Analyze Your Current Project

Use the Upgrade Assistant:

This CLI tool helps detect incompatible APIs, auto-updates your target framework, and suggests manual changes.

🧱 Step 2: Update Target Framework

Open your .csproj file and update the TargetFramework:

If you’re using multi-targeting (e.g., for libraries), update all relevant entries.

🧪 Step 3: Update Dependencies

Use the following command to update NuGet packages:

✅ Check if your third-party packages are compatible with .NET 9 or have preview builds available.

⚙️ Step 4: Update SDK Tools & Visual Studio

Ensure you're using:

• Visual Studio 2025+ (for full tooling support)

• Latest versions of:

  • .NET CLI

  • Docker Desktop (for cloud-native apps)

  • Azure Dev CLI (if deploying to cloud)

🧠 Step 5: Use .NET 9 Features (Optional But Recommended)

Now that your app runs on .NET 9, consider adopting these improvements:

🔹 For Web Apps:

• Switch to Minimal APIs for new endpoints

• Use Blazor United (hybrid server/WebAssembly rendering)

• Apply Rate Limiting middleware in ASP.NET Core

🔹 For Console/Microservices:

• Try out NativeAOT for faster, leaner executables

• Add OpenTelemetry for distributed tracing

🔹 For Libraries:

• Utilize Span<T>, Source Generators, and enhanced LINQ

🧪 Step 6: Test Thoroughly

Run tests at multiple levels:

• ✅ Unit Tests: Validate business logic

• ✅ Integration Tests: Especially important for API compatibility

• ✅ Manual Testing: UI/UX checks in Blazor, MAUI, or MVC

Use:

Also consider enabling the nullability context to make use of modern type safety:

📦 Step 7: Rebuild and Publish

Use the latest SDK to publish your app:

For AOT:

🔁 Re-test published builds, especially if switching to NativeAOT.

☁️ Step 8: Redeploy to Cloud or Server

Once everything’s tested:

• Update deployment pipelines

• Use trimmed Docker images for containerized apps

• If hosting on Azure, set runtime version in App Service configuration to .NET 9

✅ Final Checklist

🤔 Frequently Asked Questions (FAQs)

1. Can I upgrade directly from .NET 6 to .NET 9?

Yes. Although step-by-step upgrades are ideal, .NET 9 supports direct upgrade from earlier versions as long as dependencies are compatible.

2. Is NativeAOT stable in .NET 9?

Yes, NativeAOT is stable for Console and ASP.NET Core apps in .NET 9, but some scenarios (like MAUI) are still experimental.

3. Do I need to rewrite my app to benefit from .NET 9?

No. You can upgrade with minimal changes, but to fully leverage features like Blazor United or AOT, some restructuring may be needed.

4. Will Visual Studio 2022 work with .NET 9?

Partially. For full compatibility, debugging, and templates, upgrade to Visual Studio 2025 or later.

5. What’s the biggest benefit of upgrading to .NET 9?

You get improved performance, cloud-native support, smaller binaries, and access to cutting-edge features like AI integration and Blazor United.

🎯 Final Thoughts

Migrating to .NET 9 is smoother than ever thanks to the official upgrade tools and strong backward compatibility. Whether you're modernizing a legacy system or improving performance, .NET 9 offers significant returns for a relatively low migration effort.

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