Shared Worker and WasmEnableThreads

[nealles]

Hi!
I took your demo project:
https://github.com/LostBeard/BlazorWebApp10

1. I added WasmEnableThreads = true
2. I added

app.Use(async (context, next) =>
{
    context.Response.Headers.Append("Cross-Origin-Embedder-Policy", "require-corp");
    context.Response.Headers.Append("Cross-Origin-Opener-Policy", "same-origin");
    await next();
});

3. I removed inspectUri from lanchSettings.json

When I run the application Shared Worker's console shows me
VM8:6 MONO_WASM: Assert failed: SharedArrayBuffer is not enabled on this page. Please use a modern browser and set Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy http headers. See also https://aka.ms/dotnet-wasm-features Error: Assert failed: SharedArrayBuffer is not enabled on this page. Please use a modern browser and set Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy http headers. See also https://aka.ms/dotnet-wasm-features

Seems, headers are not working for the Shared Worker. Do I need to do anything special to make it work?

Upd: Also, I see an issue for a main thread:

SpawnDev.BlazorJS.lib.module.js:453 Callback invokeMethod error: 
['Invoke']
 null Error: Cannot call synchronous C# methods.
    at Dn (managed-exports.ts:308:23)
    at Object.<anonymous> (invoke-cs.ts:327:17)
    at Object.invokeDotNetFromJS (blazor.web.b9228eflpl.js:1:169260)
    at b.invokeDotNetMethod (blazor.web.b9228eflpl.js:1:4857)
    at I.invokeMethod (blazor.web.b9228eflpl.js:1:7025)
    at fn (SpawnDev.BlazorJS.lib.module.js:450:64)

[LostBeard]

Please post more of your Program.cs, not just the app.use part. The problem is probably that your ASP.Net server is only sending those headers you need for SharedArrayBuffer for your index.html page and not the service worker scripts. Your full Program.cs file contents may help me find the issue and give the exact fix.

[LostBeard]

For the Blazor Wasm apps where I need cross origin isolation so I can use SharedArrayBuffer, I sometimes use a special service worker that handles the headers. I have NOT tested it with SpawnDev.BlazorJS.WebWorkers though.

Relevant files for using service worker for COI

• https://github.com/LostBeard/SpawnDev.ILGPU/tree/master/SpawnDev.ILGPU.Demo/wwwroot
• https://github.com/LostBeard/SpawnDev.ILGPU/blob/master/SpawnDev.ILGPU.Demo/wwwroot/index.html
• https://github.com/LostBeard/SpawnDev.ILGPU/blob/master/SpawnDev.ILGPU.Demo/wwwroot/blazor-coi-serviceworker.js

Other SpawnDev Blazor Wasm data processing libraries

Depending on what you need threading for, these may be more helpful.

• SpawnDev.ILGPU - Run ILGPU C# kernels on WebGPU, WebGL, Wasm, Cuda, OpenCL, and CPU — from a single codebase.
  Write parallel compute code in C# and let the library pick the best available backend automatically. In the browser, three backends (WebGPU, WebGL, Wasm) bring GPU-accelerated compute to virtually every modern browser. On desktop and server, ILGPU's native Cuda and OpenCL backends are available alongside CPU. The same async extension methods work everywhere.

• SpawnDev.ILGPU.ML - Hardware-agnostic machine learning infrastructure for .NET. Implements high-performance neural network layers in C# that are transpiled to run on WebGPU, CUDA, OpenCL, WebGL, CPU, and Wasm via SpawnDev.ILGPU. Optimized for Blazor WebAssembly and native GPU execution.

[nealles]

Please post more of your Program.cs, not just the app.use part.

As I said I took your demo project and did only things I enumerated :) Nothing more
btw, app.use I added to server project, if it is useful, inspectUrl I removed from the server project as well,

<WasmEnableThreads>true</WasmEnableThreads>
<WasmWorkerPoolSize>4</WasmWorkerPoolSize>

I added to the client project, nothing else
And I did not add any service workers because Blazor Web App works correctly without it with WasmEnableThreads = true.

Ok, forget about Shared Web Worker, let's talk about it later, first thing I think about it is a exception in spawndev.blazorjs assembly:

SpawnDev.BlazorJS.lib.module.js:453 Callback invokeMethod error: 
['Invoke']
 null Error: Cannot call synchronous C# methods.
    at Dn (managed-exports.ts:308:23)
    at Object.<anonymous> (invoke-cs.ts:327:17)
    at Object.invokeDotNetFromJS (blazor.web.b9228eflpl.js:1:169260)
    at b.invokeDotNetMethod (blazor.web.b9228eflpl.js:1:4857)
    at I.invokeMethod (blazor.web.b9228eflpl.js:1:7025)
    at fn (SpawnDev.BlazorJS.lib.module.js:450:64)

[nealles]

I have NOT tested it with SpawnDev.BlazorJS.WebWorkers though.

Could you double-check it please because we want to use it in a close future?

[nealles]

Relevant files for using service worker for COI
https://github.com/LostBeard/SpawnDev.ILGPU/tree/master/SpawnDev.ILGPU.Demo/wwwroot
https://github.com/LostBeard/SpawnDev.ILGPU/blob/master/SpawnDev.ILGPU.Demo/wwwroot/index.html
https://github.com/LostBeard/SpawnDev.ILGPU/blob/master/SpawnDev.ILGPU.Demo/wwwroot/blazor-coi-serviceworker.js

Thanks!!!
Anyway, I will look at these links.

[LostBeard]

And as I said, please post your Program.cs. The ORDER of operations matters and without seeing your full Program.cs I can't tell if you are doing it correctly.

[nealles]

using BlazorWebApp10.Components;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents()
    .AddInteractiveWebAssemblyComponents();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseWebAssemblyDebugging();
}
else
{
    app.UseExceptionHandler("/Error", createScopeForErrors: true);
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}


app.UseStatusCodePagesWithReExecute("/not-found", createScopeForStatusCodePages: true);
app.UseHttpsRedirection();

app.Use(async (context, next) =>
{
    context.Response.Headers.Append("Cross-Origin-Embedder-Policy", "require-corp");
    context.Response.Headers.Append("Cross-Origin-Opener-Policy", "same-origin");
    await next();
});
app.UseAntiforgery();

app.MapStaticAssets();
app.MapRazorComponents<App>()
    .AddInteractiveServerRenderMode()
    .AddInteractiveWebAssemblyRenderMode()
    .AddAdditionalAssemblies(typeof(BlazorWebApp10.Client._Imports).Assembly);

app.Run();

But as I said, this issue troubles me more:

[LostBeard]

Your code is failing because your placed the middleware after the endpoints. In ASP.NET Core, once a request matches an endpoint (like a WASM file or the Shared Worker script), the pipeline is often short-circuited. By the time his code tries to append headers, the file has already been sent without them.

using BlazorWebApp10.Components;

  var builder = WebApplication.CreateBuilder(args);

  // Add services to the container.
  builder.Services.AddRazorComponents()
      .AddInteractiveServerComponents()
      .AddInteractiveWebAssemblyComponents();

  var app = builder.Build();

  // COI headers — MUST be before UseHttpsRedirection so even redirect
  // responses carry them. SharedWorkers require the script response
  // itself to have COEP, not just the creating page.
  app.Use(async (context, next) =>
  {
      context.Response.Headers.Append("Cross-Origin-Embedder-Policy", "require-corp");
      context.Response.Headers.Append("Cross-Origin-Opener-Policy", "same-origin");
      await next();
  });

  // Configure the HTTP request pipeline.
  if (app.Environment.IsDevelopment())
  {
      app.UseWebAssemblyDebugging();
  }
  else
  {
      app.UseExceptionHandler("/Error", createScopeForErrors: true);
      app.UseHsts();
  }

  app.UseStatusCodePagesWithReExecute("/not-found", createScopeForStatusCodePages: true);
  app.UseHttpsRedirection();

  app.UseAntiforgery();

  app.MapStaticAssets();
  app.MapRazorComponents<App>()
      .AddInteractiveServerRenderMode()
      .AddInteractiveWebAssemblyRenderMode()
      .AddAdditionalAssemblies(typeof(BlazorWebApp10.Client._Imports).Assembly);

  app.Run();

[nealles]

Thanks, it fixed the issue with SharedArryBuffer but I still have the issue with BlazorJS in a main thread and I can not use the Shared Worker because of this issue.

SpawnDev.BlazorJS.lib.module.js:453 Callback invokeMethod error: 
['Invoke']
 null Error: Cannot call synchronous C# methods.
    at Dn (managed-exports.ts:308:23)
    at Object.<anonymous> (invoke-cs.ts:327:17)
    at Object.invokeDotNetFromJS (blazor.web.b9228eflpl.js:1:169260)
    at b.invokeDotNetMethod (blazor.web.b9228eflpl.js:1:4857)
    at I.invokeMethod (blazor.web.b9228eflpl.js:1:7025)
    at fn (SpawnDev.BlazorJS.lib.module.js:450:64)

Upd: The issue happens even if I do not use Shared Worker so I am going to create a separate ticket for the SpawnDev.BlazorJS.
Upd2: I just tested workers in Blazor WebAssembly Standalone with the WasmEnableThreads = true and the project has the same issue, I just added builder.Services.AddWebWorkerService ();, nothing more.

using BlazorApp7;
using Microsoft.AspNetCore.Components.Web;
using Microsoft.AspNetCore.Components.WebAssembly.Hosting;
using SpawnDev.BlazorJS;
using SpawnDev.BlazorJS.WebWorkers;

var builder = WebAssemblyHostBuilder.CreateDefault(args);
builder.Services.AddBlazorJSRuntime();
builder.Services.AddWebWorkerService ();
builder.RootComponents.Add<App>("#app");
builder.RootComponents.Add<HeadOutlet>("head::after");

builder.Services.AddScoped(sp => new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });

await builder.Build().BlazorJSRunAsync();

[nealles]

I need to apologize for a thing I multiply it, but I thought, I put my demo to the wrong place because it uses workers, so I decided to place it here as well:
https://github.com/nealles/SharedWebWorkersFixing/tree/main/BlazorApp7 (let me know please if you are not able to clone the solution).
Steps to reproduce:

1. Clone the solution
2. Run it
3. When you get 100% you will see the issues in the developers console (sometimes a worker paused in Sources so it is necessary to Resume script execution.)
4. 100% will be shown forever, home page is not rendered at all

If you remove a line builder.Services.AddWebWorkerService (); it will render a home page so it happens in a workers initialization function.
Perhaps, it is connected to
var missedConnections = JS.Call<MessagePort[]>("takeOverOnConnectEvent", OnSharedWorkerConnectCallback = Callback.Create<MessageEvent>(OnSharedWorkerConnect));
since it is a sync callback but workers want async callbacks.
I tried to make 'OnSharedWorkerConnect' async and use CallbackAsync for it but it did not work unfortunately, I think I did it in a wrong way. But I believe you will fix it or suggest me a workaround to make it work.
And if you need some other info or code or anything else let me know please, I am going to send it to you without any problems.

[LostBeard]

Unfortunately, this is an issue I cannot resolve with my code. It is because of the changes Microsoft is making to the Blazor Wasm .Net runtime. I voiced my concerns on the current GitHub issue that talks about this change. I would recommend you read me post there, where I explain the issue you are having and my concerns about the direction they are taking with the .Net runtime in Blazor. If you wish, you could also post there and add your voice to the conversation. It could help. Thank you.

dotnet/aspnetcore#54365

I have suggested they add the ability to keep the .Net runtime on the main thread so my libraries can all still work even with threading is enabled or if they make running the .Net runtime in a worker the default (god forbid, required... which would kill all SpawnDev.BlazorJS libraries.)

What I suggested (does not work as of 2026-04-01):
<WasmMainThreadOnWorker>false</WasmMainThreadOnWorker>