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.

[ghost]

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 https://github.com/dotnet/runtime/blob/c768315c5698391f45a6a47d56fec4ba3df59fb8/src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/ConvertDllsToWebCil.cs#L70