c# - asp.net core中间件如何做DI？

Question

我正在尝试将依赖注入(inject)到我的中间件构造函数中，如下所示

public class CreateCompanyMiddleware
{
    private readonly RequestDelegate _next;
    private readonly UserManager<ApplicationUser> _userManager;

    public CreateCompanyMiddleware(RequestDelegate next
        , UserManager<ApplicationUser> userManager
        )
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        await _next.Invoke(context);
    }
}

我的 Startup.cs 文件看起来像

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseMySql(Configuration.GetConnectionString("IdentityConnection")));

    services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<ApplicationDbContext>()
        .AddDefaultTokenProviders();
    ...

    app.UseMiddleware<CreateCompanyMiddleware>();

    ...

但是我收到了这个错误

An error occurred while starting the application. InvalidOperationException: Cannot resolve scoped service 'Microsoft.AspNetCore.Identity.UserManager`1[Common.Models.ApplicationUser]' from root provider. Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteValidator.ValidateResolution(Type serviceType, IServiceScope scope, IServiceScope rootScope)

Answer 1

UserManager<ApplicationUser>是(默认情况下)注册为一个作用域依赖，而你的CreateCompanyMiddleware中间件在应用程序启动时构建(有效地使其成为单例)。这是一个相当标准的错误，表示您不能将作用域 依赖项带入单例 类。

在这种情况下修复很简单——你可以注入(inject) UserManager<ApplicationUser>进入你的Invoke方法:

public async Task Invoke(HttpContext context, UserManager<ApplicationUser> userManager)
{
    await _next.Invoke(context);
}

这记录在 ASP.NET Core Middleware: Per-request middleware dependencies 中:

Because middleware is constructed at app startup, not per-request, scoped lifetime services used by middleware constructors aren't shared with other dependency-injected types during each request. If you must share a scoped service between your middleware and other types, add these services to the Invoke method's signature. The Invoke method can accept additional parameters that are populated by DI: