Generic Host restricts Startup constructor injection

[Tratcher]

TLDR: The only types the generic Host supports for Startup constructor injection are IHostEnvironment, IWebHostEnvironment, and IConfiguration. Applications using WebHost are unaffected.

In 3.0 we've re-platformed the web stack onto the generic host library. You can see the change in Program.cs in the templates:

2.x:
https://github.com/aspnet/AspNetCore/blob/5cb615fcbe8559e49042e93394008077e30454c0/src/Templating/src/Microsoft.DotNet.Web.ProjectTemplates/content/EmptyWeb-CSharp/Program.cs#L20-L22
3.0:
https://github.com/aspnet/AspNetCore/blob/b1ca2c1155da3920f0df5108b9fedbe82efaa11c/src/ProjectTemplates/Web.ProjectTemplates/content/EmptyWeb-CSharp/Program.cs#L19-L24

One key behavior change here is that Host only uses one dependency injection container to build the application, as opposed to WebHost that used one for the host and one for the app. As a result the Startup constructor no longer supports custom service injection, only IHostEnvironment, IWebHostEnvironment, and IConfiguration can be injected. This change was made to avoid DI issues such as duplicate singleton services getting created.

Mitigations:

Inject services into Startup.Configure:
public void Configure(IApplicationBuilder app, IOptions<MyOptions> options)

[We'll add more based on requests for specific scenarios.]

See dotnet/aspnetcore#9337 for discussion.

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This issue has been migrated to dotnet/docs#14900