为HttpClient添加默认请求报头的四种解决方案

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");
  }
}

第四种方式

上面这种方式可以采用强类型编程方式，具体的代码如下。

class Program
{
  static Task Main()=> SendAsync4();

  private static async Task SendAsync4()
  {
    DiagnosticListener.AllListeners.Subscribe(listener =>
    {
      if (listener.Name == "HttpHandlerDiagnosticListener")
      {
        listener.SubscribeWithAdapter(new HttpClientListener());
      }
    });
      
    var httpClient = new HttpClient();
    await httpClient.GetStringAsync("http://localhost:5000/");
  }

  private sealed class HttpClientListener
  {
    [DiagnosticName("System.Net.Http.HttpRequestOut.Start")]
    public void OnSend(HttpRequestMessage request) => AddDefaultHeaders(request);

    //Must subscribute the System.Net.Http.HttpRequestOut event.
    [DiagnosticName("System.Net.Http.HttpRequestOut")]
    public void OnSend() { }
  }

  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");
  }
}

总结