依赖注入

 1.新建两个文件

public interface ICounter
{
    int Get();
}

public class Counter : ICounter
{
    int i;
    public int Get()
    {
        return i++;
    }
}

2.启动时注册

//单例模式
services.AddSingleton<ICounter, Counter>();

3.构造函数注入

    public class HomeController : Controller
    {
        private ICounter _counter;
        public HomeController(ICounter counter)
        {
            _counter = counter;
        }

        public IActionResult Index()
        {
            int count = _counter.Get();
            return Content(count.ToString());
        }

    }