Generic host: Startup class

[jmezach]

From what I can tell the generic HostBuilder currently does not have the same Startup class pattern that the WebHostBuilder uses. Personally I like the separation of concerns there since with the WebHostBuilder all the logic that deals with the host itself is maintained inside Program.cs, while the Startup class deals only with your app. In the HostBuilder model these concerns are intertwined inside Program.cs.

Was this a deliberate choice, or is it still open for discussion?

[Tratcher]

This is deliberate for two reasons.

1. Startup is ASP.NET specific, the Configure method builds a http request processing pipeline. It's not clear what a generic version of this would look like.
2. Startup requires two service containers, one from the host to instantiate it, and a second for the app with the results of ConfigureServices. This dual container model has caused a lot of confusion with the current stack so we want to avoid it in the generic stack.

That said, we're not moving web apps over to the generic host for a while yet and we'll likely need at least some compat shims for Startup when we do migrate. Here's the generic web host prototype:

Hosting/samples/GenericWebHost/Program.cs

Line 26 in e892ed8

.ConfigureWebHost((hostContext, app) =>

[davidfowl]

See #1163 (the Startup section) This was intentional.