Current implementation doesn't support at least one key scenario

[donniehale-awh]

The canonical means of registering an EF Core DbContext with the service collection is:

services.AddDbContext<MyDbContext>(o => o.UseSqlServer(configuration.GetConnectionString("MyConnectionString")));

As far as I can tell that isn't possible in the current IHostingStartup approach because the actual IConfiguration isn't available at the time that Configure(IWebHostBuilder builder) is called. When the EF Core portion of an application is (properly, IMO) implemented as its own class library, there needs to be some means of initializing the DbContext and registering it with DI.

Am I missing something (very possible)? Or is there some other supported means of doing this at application startup without the asp.net core application taking a dependency on the EF Core class library (something that we want to avoid at pretty much any cost).

Thanks.

[donniehale-awh]

In case it isn't clear, I'm referring to:

https://github.com/dotnet/aspnetcore/search?q=IHostingStartup&unscoped_q=IHostingStartup

and this documentation:

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/platform-specific-configuration?view=aspnetcore-3.1

(I started the discussion from the IHostingStartup page and thought it would be specifically about that. My apologies.)

[davidfowl]

It would look like this:

.ConfigureServices((context, services) =>
{
    services.AddDbContext<MyDbContext>(o => o.UseSqlServer(context.Configuration.GetConnectionString("MyConnectionString")));
})

[donniehale-awh]

I did a lousy job explaining the problem I'm trying to solve. My apologies.

I need to do the equivalent of ConfigureServices in an assembly other than the startup assembly. This article documents a level of supported extensibility: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/platform-specific-configuration?view=aspnetcore-3.1. Per those docs, I add a call to UseSetting in Program.cs:

public static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args) .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey, "MyAssembly"); webBuilder.UseStartup<ApiStartup>(); });

Doing that causes my assembly to get loaded. Then I place a HostingStartup assembly attribute in my assembly:

[assembly: HostingStartup(typeof(MyAssembly.DataStartup))]

And I have the DataStartup class implement IHostingStartup:

public class DataStartup : IHostingStartup. This includes implementing the interface's Configure method which is passed an IWebHostBuilder. Doing that I can use builder.ConfigureServices to do normal DI configuration.

What that does not seem to allow me to do is make the AddDbContext call where the connection string comes from configuration. That's because when the IHostingStartup.Configure(IWebHostBuilder builder) method is called, the configuration (IConfiguration) hasn't been "built" yet and made available for looking up configuration values. The startup assembly's Startup.cs class is instantiated after the configuration has been built is supplied that IConfiguration. But in this IHostingStartup approach, the configuration does not seem to be available.

I'd be happy to be corrected or shown what I'm missing. Thank you again.