“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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