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(); }); }
IMiddlewareFactory
IMiddlewareFactory提供中间件的创建方法。中间件工厂实现在容器中注册为作用域服务。
可在Microsoft.AspNetCore.Http包中找到默认的IMiddlewareFactory实现（即MiddlewareFactory）。
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