“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。
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