ConnectionString from appsettings.json in Data Tier with Entity Framework Core

You can easily add an extension method of IServiceCollection into your business/services layer and use it to register its own dependencies. Then in the startup you just call the method on the service layer without having any reference to EntityFramework in your web app.

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection.Extensions;

namespace your.service.layer
{
    public static class MyServiceCollectionExtensions
    {
        public static IServiceCollection AddMyServiceDependencies(this IServiceCollection services, string connectionString)
        {
             services.AddEntityFrameworkSqlServer()
            .AddDbContext<YourDbContext>((serviceProvider, options) =>
            options.UseSqlServer(connectionString)
                   .UseInternalServiceProvider(serviceProvider)
                   );
             return services;
        }
    }

}

Startup:

using your.service.layer;

public void ConfigureServices(IServiceCollection services)
{
    var connectionString = Configuration.GetConnectionString("EntityFrameworkConnectionString");
    services.AddMyServiceDependencies(connectionString);
}

Now your web app only needs a reference to your business/service layer and it is not directly dependent on EntityFramework.