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

Overview

If you're building high-performance web applications with .NET and WebAssembly (WASM), you've likely heard about the impressive gains Microsoft Copilot Studio achieved after upgrading their WASM engine from .NET 6 to .NET 8, and then to .NET 10. The latest version brings two standout improvements: automatic fingerprinting for WASM assets and WasmStripILAfterAOT enabled by default. These changes simplify deployment, reduce output size, and eliminate custom scripts. This tutorial walks you through migrating an existing .NET 8 WASM application to .NET 10, covering prerequisites, step-by-step instructions with concrete code examples, common pitfalls, and a summary of key takeaways.

Prerequisites

Before you begin, ensure you have the following:

• .NET 10 SDK – Download the preview or release candidate from the official .NET download page.
• An existing .NET 8 Blazor WebAssembly app (or any .NET WASM project) that you plan to upgrade.
• Familiarity with .csproj files and basic command-line usage.
• Optional: A WebWorker context if you load the WASM runtime there (we'll cover the dotnetSidecar setting).

Step-by-Step Instructions

1. Update the Target Framework

The first step is to change your project's target framework to .NET 10. Open your .csproj file and modify the TargetFramework property:

<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
  <!-- other settings remain unchanged -->
</PropertyGroup>

If you use multiple target frameworks (e.g., for Blazor WebAssembly), ensure all relevant monikers are updated.

2. Update Package References

Update all Microsoft.AspNetCore.* and Microsoft.NET.Sdk.WebAssembly.Pack references to version 10.x.x. For example:

<PackageReference Include='Microsoft.AspNetCore.Components.WebAssembly' Version='10.0.0-preview.3.*' />
<PackageReference Include='Microsoft.NET.Sdk.WebAssembly.Pack' Version='10.0.0-preview.3.*' />

Check the official NuGet feed for the latest preview or stable version.

3. Remove Custom Fingerprinting Scripts (if any)

In .NET 8, many teams implemented manual fingerprinting to enable cache busting and integrity. With .NET 10's automatic fingerprinting, you can delete those scripts. Here's what you'll typically remove:

• A PowerShell or bash script that reads blazor.boot.json, renames files with SHA256 hashes, and updates references.
• JavaScript code that passes explicit integrity arguments when fetching resources.

After deletion, rebuild and publish. The build output will now include fingerprints in filenames automatically, and dotnet.js handles integrity validation.

4. Enable WebWorker Support (if needed)

If you load the .NET WASM runtime inside a WebWorker, set dotnetSidecar: true when initializing the runtime. Here's an example in JavaScript:

const config = {
  dotnetSidecar: true,
  // other configuration
};
const { getAssemblyExports } = await DotNet.init(config);

This ensures proper initialization in a worker context.

5. Leverage WasmStripILAfterAOT (AOT Builds)

When you build with AOT (-p:PublishTrimmed=true -p:PublishAot=true), .NET 10 now strips Intermediate Language (IL) from the output by default. This reduces the final download size and improves load times. To verify, check that your build output doesn't contain both .wasm and .dll files for the same assemblies.

If you maintain a dual-engine approach (JIT for fast startup, AOT for performance), like Copilot Studio does, be aware that AOT assemblies no longer match their JIT counterparts byte-for-byte. This means deduplication of identical files between the two modes may be less effective. Plan your packaging accordingly—for example, keep separate folders for JIT and AOT, and only deduplicate truly identical files (e.g., dotnet.wasm).

6. Test Thoroughly

After migration, run your full test suite. Pay special attention to:

• Asset loading: Verify that all resources (WASM files, DLLs, satellite assemblies) are fetched correctly.
• Integrity validation: Check the browser console for any integrity errors.
• Performance: Measure startup time and steady-state execution speed compared to .NET 8.

Common Mistakes and How to Avoid Them

Mistake 1: Not Updating All Dependencies

Outdated third-party packages may cause runtime conflicts. Always run dotnet list package --outdated and update any packages that support .NET 10.

Mistake 2: Keeping Custom Fingerprinting Code

Automatic fingerprinting in .NET 10 is a drop-in replacement. If you keep your old renaming script, it will either fail (because filenames already contain hashes) or double-hash your assets. Remove it entirely.

Mistake 3: Forgetting WebWorker Configuration

If you load WASM in a worker and omit dotnetSidecar: true, the runtime may fail to initialize silently. Add this flag explicitly.

Mistake 4: Overlooking AOT Deduplication Changes

If your application ships both JIT and AOT builds (like Copilot Studio), the loss of byte-for-byte identical files means you need to adjust your packaging logic. Deduplicate only files that are genuinely identical across both modes (e.g., native assets).

Summary

Upgrading a .NET WebAssembly application to .NET 10 is straightforward: update the target framework, refresh package versions, and eliminate custom fingerprinting scripts. The automatic fingerprinting feature simplifies deployment by incorporating cache-busting and integrity validation directly into the build process. Meanwhile, WasmStripILAfterAOT shrinks AOT output sizes by default. For apps using a WebWorker, remember to set dotnetSidecar: true. By following these steps, you can unlock the same performance and maintainability gains that Copilot Studio achieved.