HttpClient在Web調用中具有廣泛的應用，而為它添加默認請求頭是我們經常遇到的需求，本文介紹4種為HttpClient添加默認請求頭的方式。

直接在創建的HttpClient對象的DefaultRequestHeaders集合中添加報頭。

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");
}
}

對于.NET Core應用來說，我們更推薦的做法是采用依賴注入的方式，利用IHttpClientFactory來創建HttpClient對象，那么我們在進行相關服務注冊的時候就可以設置默認請求報頭。

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");
}
}

由于HttpClient在發送請求的時候會利用DiagnosticSource對象發送相應的診斷事件，并且將作為請求的HttpRequestMessage對象作為請求事件內容負載。我們可以訂閱該事件，在請求被發送之前將其攔截下來，并添加相應的請求頭即可。

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");
}
}

上面這種方式可以采用強類型編程方式，具體的代碼如下。

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");
}
}

原文鏈接：https://www.cnblogs.com/artech/p/add-headers-for-httpclient.html

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