Upgrading Your .NET WebAssembly App to .NET 10: A Step-by-Step Migration Guide

Overview

If you're running a .NET WebAssembly (WASM) application in production—like Microsoft Copilot Studio—you know how critical performance and deployment efficiency are. The latest .NET 10 release brings several game-changing improvements for WASM apps, including automatic resource fingerprinting and a smarter default for AOT compilation that slashes download sizes. This guide walks you through migrating from .NET 8 to .NET 10, using Copilot Studio's real-world experience as a backdrop. By the end, you'll have a faster, leaner WASM app with simpler deployment pipelines.

We'll cover everything from updating project files to removing custom scripts you no longer need. No fluff—just practical steps and clear explanations.

Prerequisites

Before you begin, make sure you have:

• .NET 10 SDK installed (download from dotnet.microsoft.com).
• An existing .NET 8 Blazor WebAssembly app (or any .NET WASM project).
• A backup of your project (always good practice).
• Familiarity with .csproj files and basic command-line operations.
• Access to your deployment infrastructure (CI/CD pipelines, hosting environment).

Step-by-Step Migration Instructions

1. Update Target Framework Moniker

The first step is as simple as changing the target framework in your .csproj file. Open it and replace net8.0 with net10.0:

<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
</PropertyGroup>

If you have multiple project files (e.g., shared libraries), update each one. This tells the SDK to use .NET 10 APIs and tooling.

2. Update NuGet Packages

Now update all NuGet dependencies to versions compatible with .NET 10. Run this command in your solution directory:

dotnet list package --outdated

Then update each package. Pay special attention to:

• Microsoft.AspNetCore.Components.WebAssembly
• Microsoft.AspNetCore.Components.WebAssembly.DevServer
• Microsoft.NET.Sdk.WebAssembly.Pack

Use the dotnet update package command or update via your package manager. For example:

dotnet update package Microsoft.AspNetCore.Components.WebAssembly --version 10.0.0

3. Say Goodbye to Manual Fingerprinting

One of the biggest wins in .NET 10 is automatic fingerprinting. In .NET 8, you probably had something like this in your deployment pipeline:

• Read blazor.boot.json to get asset list.
• Run a PowerShell script to rename each file with a SHA256 hash.
• Pass integrity args from JavaScript when fetching resources.

You can now delete that entire custom script. Fingerprinted asset names come built-in. Review your deployment scripts and remove any manual renaming steps. If you were using a custom integrity argument in your index.html or JavaScript loader, you can remove it too.

For example, if your JavaScript included:

Blazor.start({
  resources: {
    script: {
      'dotnet.js': { integrity: 'sha256-...' }
    }
  }
});

You can now remove the integrity property entirely—dotnet.js handles it automatically.

4. Verify WebWorker Support (Bonus Tip)

If your app loads the .NET WASM runtime inside a WebWorker (to offload heavy computation), .NET 10 requires a small configuration change. When initializing, set dotnetSidecar: true:

const { dotnet } = await createDotnetRuntime({
  dotnetSidecar: true
});

This ensures proper initialization in a worker context. Without it, you may see unexpected errors.

5. Enjoy Automatic IL Stripping for AOT Builds

In .NET 10, WasmStripILAfterAOT is enabled by default for any AOT build. This means that after your .NET methods are compiled to WebAssembly, the original Intermediate Language (IL) is removed from the output. The result? Smaller download size.

If you're already using AOT, you can remove any explicit setting from your .csproj:

<!-- Remove this line if present -->
<WasmStripILAfterAOT>true</WasmStripILAfterAOT>

If you need to disable it (for debugging or special hybrid scenarios), you can explicitly set it to false—but for most production apps, the default is ideal.

6. Advanced: Dual-Engine Packaging (Copilot Studio Pattern)

Copilot Studio uses a sophisticated approach: it ships one NPM package containing both a JIT engine (fast startup) and an AOT engine (peak performance). At runtime, they load in parallel; JIT handles initial interactions, then hands off to AOT once ready. Files identical between the two modes are deduplicated to save space.

With .NET 10, note that after IL stripping, AOT assemblies no longer match their JIT counterparts bit-for-bit. This means fewer files can be deduplicated. If you use this pattern, review your deduplication logic—you may need to adjust how you package the two modes. But for a standard app, this isn't a concern.

If you're curious, Copilot Studio's approach is documented in their original post, but adopting it is optional. Most apps will be fine with a single JIT or AOT build.

Common Mistakes to Avoid

• Forgetting to update all projects: If you have a solution with multiple projects, every project targeting .NET 8 must be bumped to .NET 10, or you'll get build errors.
• Ignoring NuGet version compatibility: Some packages may not yet have a .NET 10 version. Check for preview releases or wait for stable updates.
• Keeping old fingerprinting scripts: If you leave your custom renaming script in place, it may break the new automatic fingerprinting. Delete or disable it.
• Not testing in a staging environment: Deploy to a staging URL first, especially if you use Web Workers or dual-engine setups.
• Assuming IL stripping works for all scenarios: If you rely on runtime reflection or dynamic IL generation, stripping may cause issues. Test thoroughly.

Summary

Upgrading your .NET WebAssembly app to .NET 10 is straightforward: update the target framework, refresh dependencies, and remove manual fingerprinting scripts. The automatic fingerprinting and built-in IL stripping save you time and reduce download sizes. Copilot Studio's migration proved smooth and delivered tangible benefits without breaking existing caching or validation logic.

By following these steps, you'll modernize your WASM app with minimal effort and unlock better performance for your users.