.NET点滴：说说Middleware构造中获取不到Scoped服务的问题

Posted 2022-04-22 dotNET跨平台

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。