AspNetCore React can only find spa files in dev mode

Question

My AspNetCore/React project works fine in the Development environment but as soon as I run it in any other environment I get System.InvalidOperationException: The SPA default page middleware could not return the default page '/index.html' because it was not found, and no other middleware handled the request.

The end of the startup.cs configure method looks like this

      app.UseSpa(spa =>
      {
        spa.Options.SourcePath = "ClientApp";

        if (env.IsDevelopment())
        {
          spa.UseReactDevelopmentServer(npmScript: "start");
        }
      });

but since this is identical to the corresponding code in the app generated by dotnet new react I suppose something else is upsetting it.

The webserver does start and I can access Swagger pages. It's just the app proper that can't be found. Hacking the above code so that spa.UseReactDevelopmentServer(npmScript: "start");runs causes the app to be found for all environments (environment specific config is loaded) but served by the React dev server.

I would have said this line

spa.Options.SourcePath = "ClientApp";

was what governs the finding of /index.html and friends, but something is preventing them from being found.

What else I should look at?

I've seen this The SPA default page middleware could not return the default page but I don't think this is the problem I'm facing.

A search of the project reveals than there is only one file index.html and it is located in ClientApp/public. The Configure method starts like this

  if (env.IsDevelopment())
  {
    app.UseDeveloperExceptionPage();
  }
  else
  {
    app.UseExceptionHandler("/Error");
    // The default HSTS value is 30 days...
    app.UseHsts();
  }

  app.UseHttpsRedirection();
  app.UseStaticFiles();
  app.UseSpaStaticFiles();

React is not a .Net application. These is no point to host it with IIS nor Kestrel because it's not dynamic from the server. The reason it even exists as a SPA middleware is for development purposes. — Erik Philips
– Erik Philips, Commented Apr 28, 2021 at 6:26

Peter Wone · Accepted Answer · 2021-05-04 01:36:54Z

2

React assets are not built by dotnet build or dotnet run not even with -c Release.

To also build the React assets you need dotnet publish.

This requirement may be masked by the presence of assets due to earlier use of dotnet publish. In that case they are potentially stale. Therefore, for any environment other than dev, you must use dotnet publish.

Why don't dotnet build and dotnet run build the React assets? In development you don't need them because you're using the dev server for hot swap. In production you're almost certainly using assets prepared by dotnet publish.

Why did I not remember this? I almost never do builds for environments other than dev. I don't build for production, CICD does that. Professional testers are more likely to be aware of this than programmers simply because they build for environments other than dev.

edited May 4, 2021 at 1:36

answered Apr 28, 2021 at 6:22

Peter Wone

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