. net tip: talk about the problem that the scoped service cannot be obtained in the middleware structure

Xiao Gui asked me today ：“ Why can't we use... In the constructor of middleware scope Life cycle type of ？”,

Then use examples to get the answer , Let's look at what Xiao Gui said first , It's the wrong report ：

var builder = WebApplication.CreateBuilder(args);


builder.Services.AddScoped<ITestService, TestService>();
var app = builder.Build();


app.UseMiddleware<TestMiddleware>();


app.MapGet("/djy", () =>
{
    Console.WriteLine(" get some soy sauce ！");
    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;
    // Correct posture 
    //public TestMiddleware(RequestDelegate next)
    
    public TestMiddleware(RequestDelegate next, ITestService testService)
    {
        Console.WriteLine($"Time:{DateTime.Now},ToDo:TestMiddleware.ctor");
        _next = next;
        _testService = testService;
    }
    // Correct posture 
    //public async Task InvokeAsync(HttpContext context, ITestService testService)
        
    public async Task InvokeAsync(HttpContext context)
    {
        _testService.Print();
        await _next(context);
    }
}

Reading newspapers is wrong ：

But if you take Service Injection replacement AddTransient There is no problem , Why is that ？

There is a passage on the official website ：“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. ……”, Pick the point of this passage , The middleware is initialized when the service is started , The entire lifecycle construct is called only once , and AddScoped Is the instantiation of each object ,DI Will create a , so to speak Scoped The particle size is smaller , So it can't appear in the constructor between middleware （ The construct can only be called when initializing ）. therefore Scoped Of Service Put it in InvokeAsync The parameters of the , because InvokeAsync Is called every time a request is made , and Scoped The particle size is the same , So this is ：“ Why can't we use... In the constructor of middleware scope Life cycle type of ？” The answer .