Netcore内置IOC容器，services.AddSingleton<>,
就是往内置IOC容器里配置一个单例，将接口跟实现映射在一起，
以后每次使用构造函数注入这个IProductRepository接口，
都会获得这个单例对象！ 不需要new ProductRepository()
DI --注入， IOC就是对象的容器， 配合起来就是依赖注入，按软件设计原则来说，这叫控制反转的作用
1.public interface HomeIBLL
2.public class HomeBLL : HomeIBLL
{
private readonly HomeIRepository _homeIRepository;
public HomeBLL(HomeIRepository homeIRepository)
{
_homeIRepository = homeIRepository;
}
3.public interface HomeIRepository
4.public class HomeRepository : HomeIRepository
5.services.AddSingleton<HomeIBLL, HomeBLL>();
services.AddSingleton<HomeIRepository, HomeRepository>();
6.private readonly HomeIBLL _homeIBLL;
public HomeController(ILogger<HomeController> logger, HomeIBLL homeIBLL)
{
_logger = logger;
_homeIBLL = homeIBLL;
}