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.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");
    }
}

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