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。

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.Par
576
ameter(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
ae4
();

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 requ
aec
est)
{
request.Headers.Add("x-www-foo", "123");
request.Headers.Add("x-www-bar", "456");
request.Headers.Add("x-www-baz", "789");
}
}