Asp.net-core – The type or namespace name ‘IWebHostEnvironment’ could not be found (are you missing a using directive or an assembly reference?)

For ASP.NET Core 3 release version:

   services.AddControllersWithViews().AddRazorRuntimeCompilation();

https://docs.microsoft.com/en-us/aspnet/core/mvc/views/view-compilation?view=aspnetcore-3.0

It can also be enabled conditionally only for local development, quoted from the link:

Runtime compilation can be enabled such that it's only available for local development. Conditionally enabling in this manner ensures that the published output:

Uses compiled views.
Is smaller in size.
Doesn't enable file watchers in production.

   public Startup(IConfiguration configuration, IWebHostEnvironment env)
    {
        Configuration = configuration;
        Env = env;
    }

    public IWebHostEnvironment Env { get; set; }
    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        IMvcBuilder builder = services.AddRazorPages();

#if DEBUG
            if (Env.IsDevelopment())
            {
                builder.AddRazorRuntimeCompilation();
            }
#endif
    }

There is an option in Tools -> Options that enables preview versions of the .NET Core SDK. In the VS Preview shipping channel, it is on (by default, and not settable). In the VS Release channel, it defaults to off and you can opt-in.

(Note: it's disabled in the screenshot because I have a Preview build installed.)

The reason for this is to allow you to customize whether a project using a released version of .NET Core (such as 2.1, the current LTS) will use tooling from the newer not-yet-release-quality SDK.

Prior to 16.1, this option was located on the Tools -> Options -> Projects and Solutions -> .NET Core page.