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.

[ghost]

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.