Allow empty WebApplicationBuilder to be created without default behavior

[eerhardt]

Background and Motivation

The default host builder pulls in a ton of dependencies that are rooted for trimming by default. With #32485 we added CreateSlimBuilder, which reduced the set of dependencies, but still brought some default dependencies in - for example #47664 brought in AuthN/Z and #47797 brought back in appSettings.json and logging.

We should implement the original proposal in #32485 and make a WebApplicationBuilder that doesn't have any default behavior - i.e. "empty".

Proposed API

namespace Microsoft.AspNetCore.Builder
{
    public class WebApplication
    {
       public static WebApplication Create(string[] args);
       public static WebApplicationBuilder CreateBuilder();
       public static WebApplicationBuilder CreateBuilder(string[] args);
       public static WebApplicationBuilder CreateBuilder(WebApplicationOptions options);
       public static WebApplicationBuilder CreateSlimBuilder();
       public static WebApplicationBuilder CreateSlimBuilder(string[] args);
       public static WebApplicationBuilder CreateSlimBuilder(WebApplicationOptions options);
+      public static WebApplicationBuilder CreateEmptyBuilder(WebApplicationOptions options);
    }
}

Usage Examples

WebApplicationOptions options = new() { Args = args };
WebApplicationBuilder builder = WebApplication.CreateEmptyBuilder(options);
builder.Logging.AddConsole();

WebApplication app = builder.Build();

app.UseRouting();
app.UseEndpoints(_ => { });

app.MapGet("/", () => "Hello World");

app.Run();

Features

When creating an "empty" web application, the following features will be on (✅) or off (❌) by default. Note that these features can be enabled by the app explicitly after creating the builder/application. They just won't be enabled by default.

• Features
  • StaticWebAssets ❌
  • IHostingStartup ❌
• Configuration
  • command line args ✅
  • appsetttings.json ❌
  • User secrets ❌
  • env variables ✅
• Middleware
  • Routing ❌
  • Auth ❌
  • HostFiltering ❌
  • ForwardedHeaders ❌
• Logging
  • Logging configuration support ❌
  • ConsoleLogger ❌
  • DebugLogger ❌
  • EventSourceLogger ❌
• Servers
  • IIS in proc ❌
  • IIS out of proc ❌
  • Kestrel
    • HTTP 1 ✅
    • HTTPS ❌
    • HTTP 2 ❌
    • HTTP 3 ❌

Alternative Designs

We could add all 3 overloads for CreateEmptyBuilder. But I don't see the need to make it super easy to use this API.

Risks

It may be confusing which builder to use in your app.

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[halter73]

API Review Notes

• What if I don't want to read environment variables?
  • It's hard to use environment variables from launch settings if we don't, but it would be more empty not to.
  • We could add a nullable ConfigurationManager to WebApplicationOptions. We could do that later though.
• Should Kestrel HTTP/1.1 be enabled by default?
  • We don't like forcing people to call the UseKestrelCore IHostBuilder extension method.
  • This isn't equivalent to (new HostBuilder()).ConfigureWebHost(...) which does not add a server.
  • We think this allows for H2C as well @amcasey
  • If we don't enable a server by default, you're force to call builder.Host.UseKestrelCore().
  • We could consider adding an AddKestrel IServiceCollection method later.

We want all the features listed to be ❌ except command line args which are already effectively opt-in

API Approved!

namespace Microsoft.AspNetCore.Builder
{
    public class WebApplication
    {
+      public static WebApplicationBuilder CreateEmptyBuilder(WebApplicationOptions options);
    }
}

[Tratcher]

• We could consider adding an AddKestrel IServiceCollection method later.

#48956 would also benefit from this.