如下所示的是作为下游应用的WebApp2的定义。如代码片段所示，为了验证指定的跟踪报头是否在WebApp1中被我们的组件成功转发，我们将接收到的所有请求报头拼接成一个字符串作为响应内容。
public class Program
{
public static void Main()
{
Host.CreateDefaultBuilder()
.ConfigureWebHostDefaults(web => web.Configure(app=>app.Run(Process)))
.Build()
.Run();
static Task Process(HttpContext httpContext)
{
var headerString = string.Join(";", httpContext.Request.Headers.Select(it => $"{it.Key}={it.Value}"));
return httpContext.Response.WriteAsync(headerString);
}
}
}
WebApp1的所有代码定义如下。HeaderForwarder组件通过调用IHostBuilder的扩展方法UseHeaderForwarder进行注册，在调用该方法的时候我们指定了需要转发的请求报头名称（foo和bar）。在接收到请求之后，WebApp1会利用HttpClient调用WebApp2，并将得到结果作为相应的内容。
public class Program
{
public static void Main()
{
Host.CreateDefaultBuilder()
.UseHeaderForwarder(forwarder=>forwarder.AddHeaderNames("foo", "bar"))
.ConfigureWebHostDefaults(web => web
.ConfigureServices(svcs=>svcs.AddHttpClient())
.Configure(app => app.Run(Process)))
.Build()
.Run();
static async Task Process(HttpContext httpContext)
{
var httpClient = httpContext.RequestServices.GetRequiredService<IHttpClientFactory>().CreateClient();
var headerString = await httpClient.GetStringAsync("http://localhost:6000");
await httpContext.Response.WriteAsync(headerString);
}
}
}
作为上游应用的App具有如下所示的定义。它直接利用HttpClient向WebApp1发送了一个请求，该请求携带了foo和bar这两个需要WebApp1转发的报头。如果WebApp1完成了针对这两个请求报头的转发，那么得到的响应内容将包含这两个报头的值，我们将这一验证逻辑体现在两个调试断言中。
class Program
{
static async Task Main(string[] args)
{
var httpClient = new HttpClient();
var request = new HttpRequestMessage
{
RequestUri = new Uri("http://localhost:5000"),
Method = HttpMethod.Get
};
request.Headers.Add("foo", "123");
request.Headers.Add("bar", "456");
var response = await httpClient.SendAsync(request);
var headers = (await response.Content.ReadAsStringAsync()).Split(";");
Debug.Assert(headers.Contains("foo=123"));
Debug.Assert(headers.Contains("bar=456"));
}
}
二、添加任意需要转发的请求报头
上面我们演示了HeaderForwarder组件自动提取指定的报头并自动转发的功能，实际上该组件还可以帮助我们将任意的报头添加到由HttpClient发出的请求消息中。假设WebApp1除了自动转发的foo和bar报头之外，还需要额外添加一个baz报头，我们可以对程序作如下的修改。
public class Program
{
public static void Main()
{
Host.CreateDefaultBuilder()
.UseHeaderForwarder(forwarder => forwarder.AddHeaderNames("foo", "bar"))
.ConfigureWebHostDefaults(web => web
.ConfigureServices(svcs => svcs.AddHttpClient())
.Configure(app => app.Run(Process)))
.Build()
.Run();
static async Task Process(HttpContext httpContext)
{
using (new HttpInvocationContextScope())
{
HttpInvocationContext.Current.AddOutgoingHeader("baz", "789");
var httpClient = httpContext.RequestServices.GetRequiredService<IHttpClientFactory>().CreateClient();
var headerString = await httpClient.GetStringAsync("http://localhost:6000");
await httpContext.Response.WriteAsync(headerString);
}
}
}
}
如上面的代码片段所示，我们将针对HttpClient的调用置于HttpInvocationContextScope对象构建的上下文范围中。在调用HttpClient发送请求之前，我们通过Current静态属性得到当前的HttpInvocationContext上下文，并通过调用其AddOutgoingHeader方法设置待转发的baz报头。为了验证WebApp1针对baz报头的转发，我们将App的程序进行如下的改写。
class Program
{
static async Task Main(string[] args)
{
var httpClient = new HttpClient();
var request = new HttpRequestMessage
{
RequestUri = new Uri("http://localhost:5000"),
Method = HttpMethod.Get
};
request.Headers.Add("foo", "123");
request.Headers.Add("bar", "456");
var response = await httpClient.SendAsync(request);
var headers = (await response.Content.ReadAsStringAsync()).Split(";");
Debug.Assert(headers.Contains("foo=123"));
Debug.Assert(headers.Contains("bar=456"));
Debug.Assert(headers.Contains("baz=789"));
}
}
如果涉及到多个HTTP调用都需要对相同的报头进行转发，上面介绍的这种基于HttpInvocationContextScope/HttpInvocationContext的编程模式会变得很方便。
using (new HttpInvocationContextScope())
{
HttpInvocationContext.Current
.AddOutgoingHeader("foo", "123")
.AddOutgoingHeader("bar", "456")
.AddOutgoingHeader("baz", "789");
var result1 = await httpClient.GetStringAsync("http://www.foo.com/");
var result2 = await httpClient.GetStringAsync("http://www.bar.com/");
var result3 = await httpClient.GetStringAsync("http://www.baz.com/");
}
三、在非ASP.NET Core应用中使用
在ASP.NET Core应用中，HeaderForwarder是通过调用IHostBuilder的扩展方法UseHeaderForwarder进行注册的，如果在控制台应用又该如何使用。其实很简单，HeaderForwarder针对请求（通过HttpClient发送）报头的添加是通过该注册提供的一个HttpClientObserver对象提供的，它实现了IObserver<DiagnosticListener>接口，我们只需要对该对象进行注册就可以了。
class Program
{
static async Task Main()
{
var httpClientObserver = new ServiceCollection()
.AddHeaderForwarder()
.BuildServiceProvider()
.GetRequiredService<HttpClientObserver>();
DiagnosticListener.AllListeners.Subscribe(httpClientObserver);
using (new HttpInvocationContextScope())
{
HttpInvocationContext.Current
.AddOutgoingHeader("foo", "123")
.AddOutgoingHeader("bar", "456");
var headers = (await new HttpClient().GetStringAsync("http://locahost:5000")).Split(";");
Debug.Assert(headers.Contains("foo=123"));
Debug.Assert(headers.Contains("bar=456"));
Debug.Assert(headers.Contains("baz=789"));
}
}
}
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