前言

HttpClient在Web调用中具有广泛的应用，而为它添加默认请求头是我们经常遇到的需求，本文介绍4种为HttpClient添加默认请求头的方式。下面话不多说了，来一起看看详细的介绍吧

第一种方式

直接在创建的HttpClient对象的DefaultRequestHeaders集合中添加报头。

class Program{  static Task Main()=> SendAsync1();  private static async Task SendAsync1()  {    var httpClient = new HttpClient();    AddDefaultHeaders(httpClient);    await httpClient.GetStringAsync("http://localhost:5000/");  }  private static void AddDefaultHeaders(HttpClient httpClient)  {    httpClient.DefaultRequestHeaders.Add("x-www-foo", "123");    httpClient.DefaultRequestHeaders.Add("x-www-bar", "456");    httpClient.DefaultRequestHeaders.Add("x-www-baz", "789");  }}

第二种方式

对于.NET Core应用来说，我们更推荐的做法是采用依赖注入的方式，利用IHttpClientFactory来创建HttpClient对象，那么我们在进行相关服务注册的时候就可以设置默认请求报头。

class Program{  static Task Main()=> SendAsync2();  private static async Task SendAsync2()  {    var services = new ServiceCollection();    services.AddHttpClient("", AddDefaultHeaders);    var httpClient = services      .BuildServiceProvider()      .GetRequiredService<IHttpClientFactory>()      .CreateClient();    await httpClient.GetStringAsync("http://localhost:5000/");  }  private static void AddDefaultHeaders(HttpClient httpClient)  {    httpClient.DefaultRequestHeaders.Add("x-www-foo", "123");    httpClient.DefaultRequestHeaders.Add("x-www-bar", "456");    httpClient.DefaultRequestHeaders.Add("x-www-baz", "789");  }}

第三种方式

由于HttpClient在发送请求的时候会利用DiagnosticSource对象发送相应的诊断事件，并且将作为请求的HttpRequestMessage对象作为请求事件内容负载。我们可以订阅该事件，在请求被发送之前将其拦截下来，并添加相应的请求头即可。

class Program{  static Task Main()=> SendAsync3();  private static async Task SendAsync3()  {    Func<object, HttpRequestMessage> requestAccessor = null;    DiagnosticListener.AllListeners.Subscribe(listener =>    {      if (listener.Name == "HttpHandlerDiagnosticListener")      {        listener.Subscribe(kv =>        {          if (kv.Key == "System.Net.Http.HttpRequestOut.Start")          {            requestAccessor ??= BuildRequestAccessor(kv.Value.GetType());            var request = requestAccessor(kv.Value);            AddDefaultHeaders(request);          }        });      }    });    var httpClient = new HttpClient();    await httpClient.GetStringAsync("http://localhost:5000/");    static Func<object, HttpRequestMessage> BuildRequestAccessor(Type payloadType)    {      var property = payloadType.GetProperty("Request", BindingFlags.Instance | BindingFlags.Public);      var payload = Expression.Parameter(typeof(object));      var convertedPayload = Expression.Convert(payload, payloadType);      var getRequest = Expression.Call(convertedPayload, property.GetMethod);      var convert = Expression.Convert(getRequest, typeof(HttpRequestMessage));      return Expression.Lambda<Func<object, HttpRequestMessage>>(convert, payload).Compile();    }  }  private static void AddDefaultHeaders(HttpRequestMessage request)  {    request.Headers.Add("x-www-foo", "123");    request.Headers.Add("x-www-bar", "456");    request.Headers.Add("x-www-baz", "789");  }}