注意 Build() 方法就是构建我们的请求处理管道，它返回了一个 RequestDelegate 对象，该对象实际上是一个委托对象，代表了一个处理当前请求的处理管道函数，它就是我们所谓的处理管道，以后我们将通过该委托来处理请求。
public RequestDelegate Build() { RequestDelegate app = context => { // ...... context.Response.StatusCode = StatusCodes.Status404NotFound; return Task.CompletedTask; }; foreach (var component in _components.Reverse()) { app = component(app); } return app; }
完整的 ApplicationBuilder 代码如下所示：
// Copyright (c) .NET Foundation. All rights reserved. // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information. using System; using System.Collections.Generic; using System.Linq; using System.Threading.Tasks; using Microsoft.AspNetCore.Http; using Microsoft.AspNetCore.Http.Features; using Microsoft.Extensions.Internal; namespace Microsoft.AspNetCore.Builder { public class ApplicationBuilder : IApplicationBuilder { private const string ServerFeaturesKey = "server.Features"; private const string ApplicationServicesKey = "application.Services"; private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>(); public ApplicationBuilder(IServiceProvider serviceProvider) { Properties = new Dictionary<string, object?>(StringComparer.Ordinal); ApplicationServices = serviceProvider; } public ApplicationBuilder(IServiceProvider serviceProvider, object server) : this(serviceProvider) { SetProperty(ServerFeaturesKey, server); } private ApplicationBuilder(ApplicationBuilder builder) { Properties = new CopyOnWriteDictionary<string, object?>(builder.Properties, StringComparer.Ordinal); } public IServiceProvider ApplicationServices { get { return GetProperty<IServiceProvider>(ApplicationServicesKey)!; } set { SetProperty<IServiceProvider>(ApplicationServicesKey, value); } } public IFeatureCollection ServerFeatures { get { return GetProperty<IFeatureCollection>(ServerFeaturesKey)!; } } public IDictionary<string, object?> Properties { get; } private T? GetProperty<T>(string key) { return Properties.TryGetValue(key, out var value) ? (T)value : default(T); } private void SetProperty<T>(string key, T value) { Properties[key] = value; } public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware) { _components.Add(middleware); return this; } public IApplicationBuilder New() { return new ApplicationBuilder(this); } public RequestDelegate Build() { RequestDelegate app = context => { // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened. // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware. var endpoint = context.GetEndpoint(); var endpointRequestDelegate = endpoint?.RequestDelegate; if (endpointRequestDelegate != null) { var message = $"The request reached the end of the pipeline without executing the endpoint: '{endpoint!.DisplayName}'. " + $"Please register the EndpointMiddleware using '{nameof(IApplicationBuilder)}.UseEndpoints(...)' if using " + $"routing."; throw new InvalidOperationException(message); } context.Response.StatusCode = StatusCodes.Status404NotFound; return Task.CompletedTask; }; foreach (var component in _components.Reverse()) { app = component(app); } return app; } } }
见：在 GitHub 中查看 ApplicationBuilder 源码
强类型的中间件
函数形式的中间件使用比较方便，可以直接在管道定义中使用。但是，如果我们希望能够定义独立的中间件，使用强类型的类来定义会更加方便一些。
public interface IMiddleware { public System.Threading.Tasks.Task InvokeAsync ( Microsoft.AspNetCore.Http.HttpContext context, Microsoft.AspNetCore.Http.RequestDelegate next); }
我们定义的强类型中间件可以选择实现装个接口。
next 表示请求处理管道中的下一个中间件，处理管道会将它提供给你定义的中间件。这是将各个中间件连接起来的关键。
如果当前中间件需要将请求继续分发给后继的中间件继续处理，只需要调用这个委托对象即可。否则，应用程序针对该请求的处理到此为止。
例如，增加一个可以添加自定义响应头的中间件，如下所示：