ASP.NET Core 中基于工厂的中间件激活

IMiddlewareFactory/IMiddleware 是中间件激活的扩展点。

UseMiddleware 扩展方法检查中间件的已注册类型是否实现 IMiddleware。如果是，则使用在容器中注册的 IMiddlewareFactory 实例来解析 IMiddleware 实现，而不使用基于约定的中间件激活逻辑。中间件在应用的服务容器中注册为作用域或瞬态服务。

优点：

按客户端请求（作用域服务的注入）激活
让中间件强类型化

IMiddleware 按客户端请求（连接）激活，因此作用域服务可以注入到中间件的构造函数中。

IMiddleware

IMiddleware 定义应用的请求管道的中间件。InvokeAsync(HttpContext, RequestDelegate) 方法处理请求，并返回代表中间件执行的 Task。

使用约定激活的中间件：

public class ConventionalMiddleware
{private readonly RequestDelegate _next;public ConventionalMiddleware(RequestDelegate next){_next = next;}public async Task InvokeAsync(HttpContext context, AppDbContext db){var keyValue = context.Request.Query["key"];if (!string.IsNullOrWhiteSpace(keyValue)){db.Add(new Request(){DT = DateTime.UtcNow, MiddlewareActivation = "ConventionalMiddleware", Value = keyValue});await db.SaveChangesAsync();}await _next(context);}
}

使用 MiddlewareFactory 激活的中间件：

public class FactoryActivatedMiddleware : IMiddleware
{private readonly AppDbContext _db;public FactoryActivatedMiddleware(AppDbContext db){_db = db;}public async Task InvokeAsync(HttpContext context, RequestDelegate next){var keyValue = context.Request.Query["key"];if (!string.IsNullOrWhiteSpace(keyValue)){_db.Add(new Request(){DT = DateTime.UtcNow, MiddlewareActivation = "FactoryActivatedMiddleware", Value = keyValue});await _db.SaveChangesAsync();}await next(context);}
}

程序会为中间件创建扩展：

public static class MiddlewareExtensions
{public static IApplicationBuilder UseConventionalMiddleware(this IApplicationBuilder builder){return builder.UseMiddleware<ConventionalMiddleware>();}public static IApplicationBuilder UseFactoryActivatedMiddleware(this IApplicationBuilder builder){return builder.UseMiddleware<FactoryActivatedMiddleware>();}
}

无法通过 UseMiddleware 将对象传递给工厂激活的中间件：

public static IApplicationBuilder UseFactoryActivatedMiddleware(this IApplicationBuilder builder, bool option)
{// Passing 'option' as an argument throws a NotSupportedException at runtime.return builder.UseMiddleware<FactoryActivatedMiddleware>(option);
}

将工厂激活的中间件添加到 Startup.ConfigureServices 的内置容器中：

public void ConfigureServices(IServiceCollection services)
{services.AddDbContext<AppDbContext>(options =>options.UseInMemoryDatabase("InMemoryDb"));services.AddTransient<FactoryActivatedMiddleware>();services.AddRazorPages();
}

两个中间件均在 Startup.Configure 的请求处理管道中注册：

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{if (env.IsDevelopment()){app.UseDeveloperExceptionPage();}else{app.UseExceptionHandler("/Error");}app.UseConventionalMiddleware();app.UseFactoryActivatedMiddleware();app.UseStaticFiles();app.UseRouting();app.UseEndpoints(endpoints =>{endpoints.MapRazorPages();});
}