Does not start ASP.net service with SPA (React) - asp.net

Question

Please help me launch the web service. I run the .exe file from the folder "\bin\Debug\net 5.0". When prompted https://localhost:5001/ gives an error.
I specified it in the spa.Options.sourcepath, but it didn't help.
the ERROR that I received is the following:
fail: Microsoft.AspNetCore.Server.Kestrel[13]
Connection id "0HMD866NK2DDC", Request id "0HMD866NK2DDC:00000011": An unhandled exception was thrown by the application.
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.
Your application is running in Production mode, so make sure it has been published, or that you have built your SPA manually. Alternatively you may wish to switch to the Development environment.
This is the folder structure ClientApp Paths
My config code:
public void ConfigureServices(IServiceCollection services)
{
// получаем строку подключения из файла конфигурации
string connection = Configuration.GetConnectionString("DefaultConnection");
// добавляем контекст ApplicationContext в качестве сервиса в приложение
services.AddDbContext<ApplicationContext>(options =>
options.UseSqlServer(connection));
//чтобы кирилица нормально отображалась
services.AddWebEncoders(o =>
{
o.TextEncoderSettings = new TextEncoderSettings(UnicodeRanges.BasicLatin, UnicodeRanges.Cyrillic, UnicodeRanges.CyrillicExtendedA, UnicodeRanges.CyrillicExtendedB);
});
services.AddControllersWithViews();
// In production, the React files will be served from this directory
services.AddSpaStaticFiles(configuration =>
{
configuration.RootPath = "ClientApp/build";
});
}
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Error");
// 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.UseHttpsRedirection();
app.UseStaticFiles();
app.UseSpaStaticFiles();
app.UseRouting();
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
name: "default",
pattern: "{controller}");
});
app.UseSpa(spa =>
{
spa.Options.SourcePath = "ClientApp";
if (env.IsDevelopment())
{
spa.UseReactDevelopmentServer(npmScript: "start");
}
});
}

Answer 1

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.
AspNetCore React can only find spa files in dev mode