今天小桂问我：“为什么中间件的构造函数里不能使用scope的生命周期类型啊？”，

那就用实例来得到答案吧，先看小桂说的情况，是报错的：

var builder = WebApplication.CreateBuilder(args);builder.Services.AddScoped<ITestService, TestService>();
var app = builder.Build();app.UseMiddleware<TestMiddleware>();app.MapGet("/djy", () =>
{Console.WriteLine("打酱油！");return "OK";
});app.Run();public interface ITestService
{void Print();
}
public class TestService : ITestService
{public TestService(){Console.WriteLine($"Time:{DateTime.Now},ToDo:TestService.ctor");}public void Print(){Console.WriteLine($"Time:{DateTime.Now},ToDo:TestService.Print");}
}
public class TestMiddleware
{private readonly RequestDelegate _next;private readonly ITestService _testService;//正确姿势//public TestMiddleware(RequestDelegate next)public TestMiddleware(RequestDelegate next, ITestService testService){Console.WriteLine($"Time:{DateTime.Now},ToDo:TestMiddleware.ctor");_next = next;_testService = testService;}//正确姿势//public async Task InvokeAsync(HttpContext context, ITestService testService)public async Task InvokeAsync(HttpContext context){_testService.Print();await _next(context);}
}

看报错：

但如果把Service注入换成AddTransient就没有问题，这是为什么呢？

官网有一段话：“Middleware is constructed at app startup and therefore has application life time. Scoped lifetime services used by middleware constructors aren't shared with other dependency-injected types during each request. ……”，这段话挑重点，就是中间件是服务启动时初始化，整个生命周期构造只调用一次，而AddScoped是每个对象实例化，DI就会创建一份，可以说Scoped的颗粒度更小，所以就不能在中件间的构造函数中出现（构造只有在初始化的时候调用到）。所以Scoped的Service放在InvokeAsync的参数中，因为InvokeAsync是每次请求时才调用到，和Scoped的颗粒度是一样的，所以这就是：“为什么中间件的构造函数里不能使用scope的生命周期类型啊？”的答案。