How to host an Angular app inside .NET Core 3.1 WebAPI?

Your are missing one important thing for SPA hosting:

app.UseSpa(spa =>
{
    // To learn more about options for serving an Angular SPA from ASP.NET Core,
    // see https://go.microsoft.com/fwlink/?linkid=864501
});

Handles all requests from this point in the middleware chain by returning the default page for the Single Page Application (SPA).

This middleware should be placed late in the chain, so that other middleware for serving static files, MVC actions, etc., takes precedence.

- https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.builder.spaapplicationbuilderextensions.usespa

---

Update: If you only want to map a specific route to the index.html i.e. everything starting with http://localhost/ui/ you can combine it with app.MapWhen

app.MapWhen(
    context => context.Request.Path.StartsWithSegments("/ui/", StringComparison.OrdinalIgnoreCase), 
    cfg => 
    {
        cfg.UseSpa(spa =>
        {
            // To learn more about options for serving an Angular SPA from ASP.NET Core,
            // see https://go.microsoft.com/fwlink/?linkid=864501
        });
    }
);

I had the same problem but app.UseSpa(...) did not work as I guess I was missing some dependecies.

Alternativele you could add endpoints.MapFallbackToFile("/index.html"); in app.UseEndpoints(..), which is part of the Microsoft.AspNetCore.StaticFiles assembly.

So it would look like this:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllers();
    endpoints.MapFallbackToFile("/index.html");
});

Source: https://weblog.west-wind.com/posts/2020/Jul/12/Handling-SPA-Fallback-Paths-in-a-Generic-ASPNET-Core-Server#server-side-navigation-of-client-side-routes