WasmEnableThreads incrementalism is broken

[SteveSandersonMS]

Description

If you have a Blazor WebAssembly app with WasmEnableThreads=true, and then change it to false, the build output is corrupted and won't work until you delete bin/obj and build again.

Reproduction Steps

1. Create a .NET 9 Blazor WebAssembly app with WasmEnableThreads=true. Make sure you set this flag before you build for the first time.
2. Run it from VS with Ctrl+F5 and observe it working
3. Change WasmEnableThreads to false
4. Ctrl+F5 again

Expected behavior

It should work in single-threaded mode

Actual behavior

It fails with:

Regression?

No

Known Workarounds

Delete bin & obj folders

(which unfortunately is not something we can expect developers to do in reality)

Configuration

.NET 9 WebAssembly

Other information

No response

[SteveSandersonMS]

cc @lewing @pavelsavara

[SteveSandersonMS]

Note that a similar problem also happens when switching WasmEnableThreads from false to true without deleting bin/obj, which is even more confusing because then you will likely think that further config is required to enable threading, when it isn't.

Tagging subscribers to 'arch-wasm': @lewing
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

If you have a Blazor WebAssembly app with WasmEnableThreads=true, and then change it to false, the build output is corrupted and won't work until you delete bin/obj and build again.

Reproduction Steps

1. Create a .NET 9 Blazor WebAssembly app with WasmEnableThreads=true. Make sure you set this flag before you build for the first time.
2. Run it from VS with Ctrl+F5 and observe it working
3. Change WasmEnableThreads to false
4. Ctrl+F5 again

Expected behavior

It should work in single-threaded mode

Actual behavior

It fails with:

Regression?

No

Known Workarounds

Delete bin/obj (which unfortunately is not something we can expect developers to do in reality)

Configuration

.NET 9 WebAssembly

Other information

No response

Author:	SteveSandersonMS
Assignees:	-
Labels:	arch-wasm, untriaged, area-Build-mono, os-browser
Milestone:	-

[lewing]

Thanks for pointing this out, we'll make sure it gets resolved

[maraf]

The problem is with conversion to webcil format. The ConvertDllsToWebCil compares only file timestamp, but not if the original location has changed

runtime/src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/ConvertDllsToWebCil.cs

Line 70 in c768315

if (Utils.IsNewerThan(dllFilePath, finalWebcil))