前言
在之前的文章我们介绍过HttpClient相关的服务发现，确实HttpClient是目前.NET Core进行Http网络编程的的主要手段。在之前的介绍中也看到了，我们使用了一个很重要的抽象HttpMessageHandler，接下来我们就探究一下HttpClient源码，并找寻它和HttpMessageHandler的关系究竟是怎么样的。
HttpClient源码解析
首先我们找到HttpClient源码的位置，微软也提供了专门的网站可以查找.Net Core源码有兴趣的同学可以自行查阅。接下来我们查阅一下HttpClient的核心代码。首先，我们可以看到HttpClient继承自HttpMessageInvoker这个类，待会我们在探究这个类。
public class HttpClient : HttpMessageInvoker { } 然后我们看下几个核心的构造函数
public HttpClient() : this(new HttpClientHandler()) { } public HttpClient(HttpMessageHandler handler) : this(handler, true) { } public HttpClient(HttpMessageHandler handler, bool disposeHandler) : base(handler, disposeHandler) { _timeout = s_defaultTimeout; _maxResponseContentBufferSize = HttpContent.MaxBufferSize; _pendingRequestsCts = new CancellationTokenSource(); } 通过这几个构造函数我们看出，我们可以传递自定义的HttpMessageHandler。我们再看无参默认的构造，其实也是实例化了HttpClientHandler传递给了自己的另一个构造函数，我们之前讲解过HttpClientHandler是继承自了HttpMessageHandler，通过最后一个构造函数可知最终HttpMessageHandler，传给了父类HttpMessageInvoker。到了这里我们基本上就可以感受到HttpMessageHandler在HttpClient中存在的意义。
接下来，我们从一个最简单，而且最常用的方法为入口开始探索HttpClient的工作原理。这种方式可能是我们最常用而且最有效的的探索源码的方式了。个人建议没看过源码，或者刚开始入门看源码的小伙伴们，找源码的入口一定是你最有把握的的一个，然后逐步深入了解。接下来我们选用HttpClient的GetAsync开始入手，而且是只传递Url的那一个。
public Task<HttpResponseMessage> GetAsync(string? requestUri) { return GetAsync(CreateUri(requestUri)); } public Task<HttpResponseMessage> GetAsync(Uri? requestUri) { return GetAsync(requestUri, defaultCompletionOption); } 通过这里我们可以大致了解到。其实大部分最简单的调用方式，往往都是从最复杂的调用方式，一步步的封装起来的，只是系统帮我们初始化了一部分参数，让我们按需使用。顺着方法一直向下找，最后找到了这里。
public Task<HttpResponseMessage> GetAsync(Uri? requestUri, HttpCompletionOption completionOption, CancellationToken cancellationToken) { return SendAsync(CreateRequestMessage(HttpMethod.Get, requestUri), completionOption, cancellationToken); } 由此可以看出这里是所有GetAsync方法的执行入口，我们通过查找SendAsync引用可以发现。不仅仅是GetAsync, PostAsync，PutAsync，DeleteAsync最终都是调用了这个方法。也就是说SendAsync是所有发送请求的真正执行者。接下来我们就查看SendAsync方法，部分边角料代码我粘贴的时候将会做删减。
public Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationToken cancellationToken) { if (request == null) { throw new ArgumentNullException(nameof(request)); } CheckDisposed(); CheckRequestMessage(request); SetOperationStarted(); //这里会把发送请求的HttpRequestMessage准备妥当 PrepareRequestMessage(request); CancellationTokenSource cts; bool disposeCts; bool hasTimeout = _timeout != s_infiniteTimeout; long timeoutTime = long.MaxValue; if (hasTimeout || cancellationToken.CanBeCanceled) { disposeCts = true; cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _pendingRequestsCts.Token); if (hasTimeout) { timeoutTime = Environment.TickCount64 + (_timeout.Ticks / TimeSpan.TicksPerMillisecond); cts.CancelAfter(_timeout); } } else { disposeCts = false; cts = _pendingRequestsCts; } Task<HttpResponseMessage> sendTask; try { //***这里是核心，最终执行调用的地方！！！ sendTask = base.SendAsync(request, cts.Token); } catch (Exception e) { HandleFinishSendAsyncCleanup(cts, disposeCts); if (e is OperationCanceledException operationException && TimeoutFired(cancellationToken, timeoutTime)) { throw CreateTimeoutException(operationException); } throw; } //这里处理输出的唯一类型HttpResponseMessage return completionOption == HttpCompletionOption.ResponseContentRead && !string.Equals(request.Method.Method, "HEAD", StringComparison.OrdinalIgnoreCase) ? FinishSendAsyncBuffered(sendTask, request, cts, disposeCts, cancellationToken, timeoutTime) : FinishSendAsyncUnbuffered(sendTask, request, cts, disposeCts, cancellationToken, timeoutTime); } 通过分析这段代码可以得知，HttpClient类中最终执行的是父类的SendAsync的方法。看来是时候查看父类HttpMessageInvoker的源码了。
HttpMessageInvoker源码解析
public class HttpMessageInvoker : IDisposable { private volatile bool _disposed; private readonly bool _disposeHandler; private readonly HttpMessageHandler _handler; public HttpMessageInvoker(HttpMessageHandler handler) : this(handler, true) { } public HttpMessageInvoker(HttpMessageHandler handler, bool disposeHandler) { if (NetEventSource.IsEnabled) NetEventSource.Enter(this, handler); if (handler == null) { throw new ArgumentNullException(nameof(handler)); } if (NetEventSource.IsEnabled) NetEventSource.Associate(this, handler); _handler = handler; _disposeHandler = disposeHandler; if (NetEventSource.IsEnabled) NetEventSource.Exit(this); } public virtual Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { if (request == null) { throw new ArgumentNullException(nameof(request)); } CheckDisposed(); if (NetEventSource.IsEnabled) NetEventSource.Enter(this, request); //***这里是HttpClient调用的本质，其实发送请求的根本是HttpMessageHandler的SendAsync Task<HttpResponseMessage> task = _handler.SendAsync(request, cancellationToken); if (NetEventSource.IsEnabled) NetEventSource.Exit(this, task); return task; } public void Dispose() { Dispose(true); GC.SuppressFinalize(this); } protected virtual void Dispose(bool disposing) { if (disposing && !_disposed) { _disposed = true; if (_disposeHandler) { _handler.Dispose(); } } } private void CheckDisposed() { if (_disposed) { throw new ObjectDisposedException(GetType().ToString()); } } } 是的，你并没有看错，整个HttpMessageInvoker就这么多代码，而且还是靠子类初始化过来的基本属性。找到SendAsync方法，这里基本上可以总结一点，负责调用输入输出的类只有两个。一个是提供请求参数的HttpRequestMessage，另一个是接收输出的HttpResponseMessage。这里也给我们日常工作编码中提供了一个很好的思路。针对具体某个功能的操作方法，最好只保留一个，其外围调用，都是基于该方法的封装。然后我们找到了发送请求的地方_handler.SendAsync(request, cancellationToken)，而handler正是我们通过HttpClient传递下来的HttpMessageHandler.由此可知，HttpClient的本质是HttpMessageHandler的包装类。
自定义HttpClient
探究到这里我们也差不多大概了解到HttpClient类的本质是什么了。其实到这里我们可以借助HttpMessageHandler的相关子类，封装一个简单的Http请求类.接下来我将动手实现一个简单的Http请求类，我们定义一个类叫MyHttpClient,实现代码如下
public class MyHttpClient : IDisposable { private readonly MyHttpClientHandler _httpClientHandler; private readonly bool _disposeHandler; private volatile bool _disposed; public MyHttpClient() :this(true) { } public MyHttpClient(bool disposeHandler) { _httpClientHandler = new MyHttpClientHandler(); _disposeHandler = disposeHandler; } public Task<HttpResponseMessage> GetAsync(string url) { return GetAsync(new Uri(url)); } public Task<HttpResponseMessage> GetAsync(Uri uri) { HttpRequestMessage httpRequest = new HttpRequestMessage { Method = HttpMethod.Get, RequestUri = uri }; return SendAsync(httpRequest,CancellationToken.None); } public Task<HttpResponseMessage> PostAsync(string url, HttpContent content) { return PostAsync(new Uri(url),content,null); } public Task<HttpResponseMessage> PostAsync(Uri uri, HttpContent content,Dictionary<string,string> headers) { HttpRequestMessage httpRequest = new HttpRequestMessage { Method = HttpMethod.Post, RequestUri = uri, Content = content }; if (headers != null && headers.Any()) { foreach (var head in headers) { httpRequest.Headers.Add(head.Key,head.Value); } } return SendAsync(httpRequest, CancellationToken.None); } private Task<HttpResponseMessage> SendAsync(HttpRequestMessage httpRequest, CancellationToken cancellationToken) { if (httpRequest.RequestUri == null || string.IsNullOrWhiteSpace(httpRequest.RequestUri.OriginalString)) { throw new ArgumentNullException("RequestUri"); } return _httpClientHandler.SendRequestAsync(httpRequest, cancellationToken); } public void Dispose() { Dispose(true); GC.SuppressFinalize(this); } protected virtual void Dispose(bool disposing) { if (disposing && !_disposed) { _disposed = true; if (_disposeHandler) { _httpClientHandler.Dispose(); } } } } 由于HttpMessageHandler的SendAsync是protected非子类无法直接调用，所以我封装了一个MyHttpClientHandler继承自HttpClientHandler在MyHttpClient中调用，具体实现如下
public class MyHttpClientHandler : HttpClientHandler { public Task<HttpResponseMessage> SendRequestAsync(HttpRequestMessage request, CancellationToken cancellationToken) { return this.SendAsync(request, cancellationToken); } protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { return base.SendAsync(request, cancellationToken); } } 最后写了一段测试代码
using (MyHttpClient httpClient = new MyHttpClient()) { Task<HttpResponseMessage> httpResponse = httpClient.GetAsync("http://localhost:5000/Person/GetPerson?userId=1"); HttpResponseMessage responseMessage = httpResponse.Result; if (responseMessage.StatusCode == HttpStatusCode.OK) { string content = responseMessage.Content.ReadAsStringAsync().Result; if (!string.IsNullOrWhiteSpace(content)) { System.Console.WriteLine(content); } } } 到这里自己实现MyHttpClient差不多到此结束了，因为只是讲解大致思路，所以方法封装的相对简单，只是封装了Get和Post相关的方法。
总结
通过本文分析HttpClient的源码，我们大概知道了HttpClient本质还是HttpMessageHandler的包装类。最终的发送还是调用的HttpMessageHandler的SendAsync方法。最后，我根据HttpClientHandler实现了一个MyHttpClient。以上只是本人理解，如果处在理解不正确或者不恰当的地方，望多多包涵，同时也期望能指出理解不周的地方。我写文章的主要一部分是想把我的理解传递给大家，欢迎大家多多交流。
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