介绍：
HttpClient用于发送http请求，默认情况下内部使用HttpWebRequest发送请求，这一行为可以通过构造函数中指定HttpMessageHandler参数来使用不同的通道
HttpClient这个类本身并不会进行实际的网络请求收发处理，我们应将其理解成一个容器、一个中继者，实际的网络请求核心在HttpClientHanlder中，也就是下面图中对应的Inner Handler。
HttpClient构造函数可以接受一个HttpMessageHandler类型的参数
HttpClientHandler：HttpClientHandler继承自HttpMessageHandler，用于设置cookie、代理、证书、Headlers等属性
DelegatingHandler：抽象类，DelegatingHandler继承自HttpMessageHandler，DelegatingHandler内部有一个属性InnerHandler，http请求都是通过InnerHandler发送。通过继承此类可实现类似HttpModule的功能
HttpClient继承自HttpMessageInvoker，从名字可以看出HttpClient是一个调用器，它是对HttpMessageHandler的一个调用，HttpClient本身不提供Http请求功能
HttpClient请求过程：
从Request发起，经过DelegatingHanlder处理后，进入InnerHandler，数据返回后再从Inner Handler 返回到Delegating Hanlder进行处理，最后返回结果。
从设计角度来讲，HttpClient库提供了强大的扩展性，使用者不需要任何继承即可完成对HttpClient的扩展（如果对设计模式熟悉，可以清楚的看出这里用到了装饰器模式）
Httpclient的设计图：
案例1：
设置cookie、Headers、代理、证书等，通过设置HttpClientHandler属性实现
HttpClientHandler httpClientHandler = new HttpClientHandler(); httpClientHandler.CookieContainer.Add(new Cookie("id", "1")); //httpClientHandler.Proxy = null; //httpClientHandler.Credentials = null; HttpClient httpClient = new HttpClient(httpClientHandler);
案例2：日志
将HttpClient请求、响应内容记录日志，通过继承DelegatingHandler抽象类实现
public class LoggingHandler : DelegatingHandler { public LoggingHandler(HttpMessageHandler handler) : base(handler) { } protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { if (request.Content!=null) { Debug.WriteLine(await request.Content.ReadAsStringAsync()); } HttpResponseMessage response = await base.SendAsync(request, cancellationToken); if (response.Content!=null) { Debug.WriteLine(await response.Content.ReadAsStringAsync()); } return response; } }
HttpMessageHandler handler = new HttpClientHandler(); HttpClient client = new HttpClient(new LoggingHandler(handler));
代码说明：
1、HttpClient这个类本身并不会进行实际的网络请求收发处理，我们应将其理解成一个容器、一个中继者，实际的网络请求核心在HttpClientHanlder中，也就是前面图中对应的Inner Handler。
2、我们自己定义了一个LoggingHandler，这个类对应Delegating Handler 是我们自定义的、装饰在Inner Handler外的Handler。
3、DelegatingHandler重载了SendAsync，在其内部调用了InnerHandler的SendAsync方法，如此我们便可以在实际请求发出，以及返回后进行各种统一的处理，总结起来仍是上面图中画出的，逐层调用。
4、HttpClientHandler、DelegatingHandler都是继承自HttpMessageHandler
5、必须给LoggingHandler传入HttpClientHandler，因为最终都是通过HttpClientHandler发送请求，如果不传会抛异常
HttpClientHandler：
public class HttpClientHandler : HttpMessageHandler {}
DelegatingHandler：
public abstract class DelegatingHandler : HttpMessageHandler {}
案例3：重试
封装一个RetryHandler，目的是失败重试
RetryHandler：
public class RetryHandler : DelegatingHandler { private const int MAX_COUNT = 3; protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { HttpResponseMessage response = null; for (int i = 0; i < MAX_COUNT; i++) { response = await base.SendAsync(request, cancellationToken); if (response.IsSuccessStatusCode) { return response; } } return response; } }
HttpMessageHandler handler = new HttpClientHandler(); handler = new LoggingHandler(handler); handler = new RetryHandler(handler); HttpClient client = new HttpClient(handler);
案例4：ASP.NET Core中指定DelegatingHandler、HttpClientHandler
public void ConfigureServices(IServiceCollection services) { services.AddHttpClient("client1", client => { client.BaseAddress = new System.Uri("https://www.baidu.com"); }) .AddHttpMessageHandler(() => new RetryHandler())//设置DelegatingHandler .ConfigurePrimaryHttpMessageHandler(() => new System.Net.Http.HttpClientHandler() {//设置HttpClientHandler，也就是InnerHandler UseCookies = false }); }
HttpResponseMessage：
HttpClient请求的返回类型，内部包含Headlers、Content、StatusCode、IsSuccessStatusCode等属性
其中Content属性是HttpContent类型，可转成对应类型，获取Content数据
性能隐患：
HttpClient有一个性能隐患，当要发送大量的http请求，如果频繁的创建HttpClient对象，会频繁建立TCP连接，连接不够用时会出现timeout，所以应该避免频繁创建HttpClient对象，如下使用静态变量+长连接。
private static readonly HttpClient _httpClient;
static ApiLogger()
{
_httpClient = new HttpClient();
_httpClient.Timeout = new TimeSpan(0, 0, 10);
_httpClient.DefaultRequestHeaders.Connection.Add("keep-alive");
}
上面这种方式有一个缺点，会缓存Dns，所以.net core中应该使用HttpClientFactory创建HttpClient
HttpClient扩展方法：
public static class HttpClientEx
{
//GZIP压缩
//var handler = new HttpClientHandler() { AutomaticDecompression = DecompressionMethods.GZip }
//HttpClient client = new HttpClient(handler);
/// <summary>
/// http://www.it165.net/pro/html/201306/6052.html
/// GET获取HTTP信息
/// </summary>
/// <param name="url"></param>
/// <returns></returns>
public static string Get(this HttpClient client,string url)
{
return client.GetStringAsync(url).Result;
}
/// <summary>
/// POST空的HTTP信息
/// http://www.cnblogs.com/xishuai/p/aspnet_mvc_web_api_httpclient_json_frombody_post.html
/// </summary>
/// <param name="url"></param>
/// <returns></returns>
public static string Post(this HttpClient client,string url)
{
string responseText = null;
using (StringContent content = new StringContent(string.Empty))
{
content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");
using (HttpResponseMessage responseMessage = client.PostAsync(url, content).Result)
{
responseMessage.EnsureSuccessStatusCode();//确保请求成功
responseText = responseMessage.Content.ReadAsStringAsync().Result;
}
}
return responseText;
}
/// <summary>
/// http://www.haogongju.net/art/1642652
/// POST方式发送信息(x-www-form-urlencoded字典格式)
/// </summary>
/// <param name="url"></param>
/// <param name="dict">x-www-form-urlencoded字典格式</param>
/// <param name="isException">false表示抛出异常，true表示返回异常错误信息</param>
/// <returns></returns>
public static string Post(this HttpClient client,string url, Dictionary<string, string> dict)
{
string responseText = null;
using (FormUrlEncodedContent content = new FormUrlEncodedContent(dict))
{
//content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");
using (HttpResponseMessage responseMessage = client.PostAsync(url, content).Result)
{
responseMessage.EnsureSuccessStatusCode();
responseText = responseMessage.Content.ReadAsStringAsync().Result;
}
}
return responseText;
}
/// <summary>
/// http://www.cnblogs.com/xishuai/p/aspnet_mvc_web_api_httpclient_json_frombody_post.html
/// POST方式发送信息(raw文本格式)
/// </summary>
/// <param name="url"></param>
/// <param name="raw">raw文本格式</param>
/// <param name="isException">false表示抛出异常，true表示返回异常错误信息</param>
/// <returns></returns>
public static string Post(this HttpClient client,string url, string raw)
{
string responseText = null;
using (StringContent content = new StringContent(raw))
{
content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
using (HttpResponseMessage responseMessage = client.PostAsync(url, content).Result)
{
responseMessage.EnsureSuccessStatusCode();
responseText = responseMessage.Content.ReadAsStringAsync().Result;
}
}
return responseText;
}
}
在.NET Core中，如果使用HttpClientFactory创建HttpClient，如何添加HttpMessageHandler？
需要自己去实现接口IHttpMessageHandlerBuilderFilter，看下一篇
参考：
https://www.cnblogs.com/Herzog3/p/6128822.html
https://www.cnblogs.com/Leo_wl/p/3439512.html
https://docs.microsoft.com/en-us/aspnet/web-api/overview/advanced/http-message-handlers
https://blog.csdn.net/weixin_30236595/article/details/101066107
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