可以注册 IHttpClientFactory 并将其用于配置和创建应用中的 HttpClient 实例。 这能带来以下好处：
- 提供一个中心位置，用于命名和配置逻辑
HttpClient实例。 例如，可以注册 github 客户端，并将它配置为访问 GitHub。 可以注册一个默认客户端用于其他用途。 - 通过委托
HttpClient中的处理程序整理出站中间件的概念，并提供适用于基于 Polly 的中间件的扩展来利用概念。 - 管理基础
HttpClientMessageHandler实例的池和生存期，避免在手动管理HttpClient生存期时出现常见的 DNS 问题。 - （通过
ILogger）添加可配置的记录体验，以处理工厂创建的客户端发送的所有请求。
在应用中可以通过以下多种方式使用 IHttpClientFactory
基本用法
???????public void ConfigureServices(IServiceCollection services) ???????{ ???????????services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2); ???????????services.AddHttpClient(); ???????} ???[Route("api/[controller]")] ???[ApiController] ???public class ValuesController : ControllerBase ???{ ???????private readonly IHttpClientFactory _clientFactory; ???????public ValuesController(IHttpClientFactory clientFactory) ???????{ ???????????_clientFactory = clientFactory; ???????} ???????// GET api/values ???????[HttpGet] ???????public async Task<string> Get() ???????{ ???????????HttpClient client = _clientFactory.CreateClient(); ???????????//方法一: ???????????//HttpRequestMessage request = new HttpRequestMessage ???????????//{ ???????????// ???Method = new HttpMethod("get"), ???????????// ???RequestUri = new System.Uri("http://localhost:5000/api/values"), ???????????//}; ???????????//HttpResponseMessage response = await client.SendAsync(request); ???????????//string res = await response.Content.ReadAsStringAsync(); ???????????//return res; ???????????//方法二: ???????????string res = await client.GetStringAsync("http://localhost:5000/api/values"); ???????????return res; ???????} ????}命名客户端
???????public void ConfigureServices(IServiceCollection services) ???????{ ???????????services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2); ???????????services.AddHttpClient("test", c => ???????????{ ???????????????c.BaseAddress = new Uri("http://localhost:5000"); ???????????}); ???????} ???????public async Task<string> Get() ???????{ ???????????HttpClient client = _clientFactory.CreateClient("test"); ???????????//注册名叫 "test" 的客户端时,已经指定了该客户端的请求基地址,所以这里不需要指定主机名了 ???????????return await client.GetStringAsync("api/values"); ???????}类型化客户端
???public class TestHttpClient ???{ ???????public HttpClient Client { get; set; } ???????public TestHttpClient(HttpClient client) ???????{ ???????????client.BaseAddress = new System.Uri("http://localhost:5000"); ???????????Client = client; ???????} ???????public async Task<string> Get() ???????{ ???????????return await Client.GetStringAsync("api/values"); ???????} ???} ???????public void ConfigureServices(IServiceCollection services) ???????{ ???????????services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2); ???????????services.AddHttpClient<TestHttpClient>(c => ???????????{ ???????????????//可以在这里设置,也可以在构造函数设置. ???????????????//c.BaseAddress = new System.Uri("http://localhost:5000"); ???????????}); ???????} ???[Route("api/[controller]")] ???[ApiController] ???public class ValuesController : ControllerBase ???{ ???????private readonly TestHttpClient _client; ???????public ValuesController(TestHttpClient client) ???????{ ???????????_client = client; ???????} ????????????[HttpGet] ???????public async Task<string> Get() ???????{ ???????????return await _client.Get(); ???????} ???}出站请求中间件
HttpClient 已经具有委托处理程序的概念，这些委托处理程序可以链接在一起，处理出站 HTTP 请求。 IHttpClientFactory 可以轻松定义处理程序并应用于每个命名客户端。 它支持注册和链接多个处理程序，以生成出站请求中间件管道。 每个处理程序都可以在出站请求前后执行工作。 此模式类似于 ASP.NET Core 中的入站中间件管道。 它提供了一种用于管理围绕 HTTP 请求的横切关注点的机制，包括缓存、错误处理、序列化以及日志记录。
要创建处理程序，需要定义一个派生自 DelegatingHandler 的类。 重写 SendAsync 方法，在将请求传递至管道中的下一个处理程序之前执行代码：
???public class ValidateHeaderHandler : DelegatingHandler ???{ ???????protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) ???????{ ???????????if (!request.Headers.Contains("refuge")) ???????????{ ???????????????return new HttpResponseMessage(HttpStatusCode.BadRequest) ???????????????{ ???????????????????Content = new StringContent("not found refuge") ???????????????}; ???????????} ???????????return await base.SendAsync(request, cancellationToken); ???????} ???} ???????public void ConfigureServices(IServiceCollection services) ???????{ ???????????services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2); ???????????services.AddTransient<ValidateHeaderHandler>();//生存期必须是临时 ???????????services.AddHttpClient("test", c => ???????????????{ ???????????????????c.BaseAddress = new Uri("http://localhost:5000"); ???????????????}) ???????????????.AddHttpMessageHandler<ValidateHeaderHandler>(); ???????}HttpClient 和生存期管理
每次对 IHttpClientFactory 调用 CreateClient 都会返回一个新 HttpClient 实例:
???????public IEnumerable<int> Get() ???????{ ???????????//测试生存期 ???????????for (int i = 0; i < 4; i++) ???????????{ ???????????????HttpClient client = i % 2 == 0 ???????????????????? _clientFactory.CreateClient("test") ???????????????????: _clientFactory.CreateClient(); ???????????????yield return client.GetHashCode(); ???????????} ???????}CreateClient 方法内部会调用 CreateHandler 方法,后者创建 HttpMessageHandler,
源码如下:
???public HttpClient CreateClient(string name) ???{ ?????if (name == null) ???????throw new ArgumentNullException(nameof (name)); ?????HttpClient httpClient = new HttpClient(this.CreateHandler(name), false); ?????HttpClientFactoryOptions clientFactoryOptions = this._optionsMonitor.Get(name); ?????for (int index = 0; index < clientFactoryOptions.HttpClientActions.Count; ++index) ???????clientFactoryOptions.HttpClientActions[index](httpClient); ?????return httpClient; ???} ???public HttpMessageHandler CreateHandler(string name) ???{ ?????if (name == null) ???????throw new ArgumentNullException(nameof (name)); ?????ActiveHandlerTrackingEntry entry = this._activeHandlers.GetOrAdd(name, this._entryFactory).Value; ?????this.StartHandlerEntryTimer(entry); ?????return (HttpMessageHandler) entry.Handler; ???}而这个 _activeHandlers 的类型是 :
一个线程安全的键值对集合.
因此,实际上创建的 HttpMessageHandler 实例会汇集到池中.新建 HttpClient 实例时,可能会重用池中的 HttpMessageHandler 实例（如果生存期尚未到期的话）.
由于每个处理程序通常管理自己的基础 HTTP 连接,因此需要池化处理程序.创建超出必要数量的处理程序可能会导致连接延迟. 部分处理程序还保持连接无期限地打开,这样可以防止处理程序对 DNS 更改作出反应.
处理程序的默认生存期为两分钟,可在每个命名客户端上重写默认值:
services.AddHttpClient("test").SetHandlerLifetime(TimeSpan.FromMinutes(5));配置 HttpMessageHandler
有时候,我们需要控制客户端使用的内部 HttpMessageHandler .
???????????services.AddHttpClient("test") ???????????????.ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler() ???????????????{ ???????????????????AllowAutoRedirect = false, ???????????????});ASP.NET Core 2.2 基础知识(十二) 发送 HTTP 请求
原文地址：https://www.cnblogs.com/lonelyxmas/p/10230908.html