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", "7");
 }
}

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", "7");
 }
}

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", "7");
 }
}

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", "7");
 }
}

总结