在相同的Asp.Net Core2解决方案中将DbContext注入类库项目

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

namespace YourLibraryProject
{
    public static class ServiceCollectionExtensions
    {
        public static IServiceCollection AddLibraryDbContext(this IServiceCollection services, string connectionString)
        {
            services.AddDbContext<YourDbContext>(options =>
                options.UseSqlServer(connectionString));

            return services;
        }
    }
}

using YourLibraryProject;

namespace YourMainProject
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            // 其他服务的注册...

            services.AddLibraryDbContext(Configuration.GetConnectionString("YourConnectionString"));

            // 其他服务的注册...
        }
    }
}

{
  "ConnectionStrings": {
    "YourConnectionString": "YourDatabaseConnectionString"
  }
}