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Showing content from https://github.com/dotnet/docs/issues/14900 below:

Generic Host restricts Startup constructor injection · Issue #14900 · dotnet/docs · GitHub

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.

Version introduced

3.0

Old behavior

The Startup constructor could constructor inject arbitrary types.

New behavior

The following types can be injected - IHostEnvironment, IWebHostEnvironment, and IConfiguration

Reason for change

This is a consequence of re-platting the web stack onto the generic host library.

Recommended action

You may mitigate this by inject services into Startup.Configure. For example
public void Configure(IApplicationBuilder app, IOptions<MyOptions> options)