public void ConfigureServices(IServiceCollection services)
{services.AddControllers();services.AddTransient<LogHandler>();services.AddTransient<RetryHandler>();var googleLocation = Configuration["Google"];services.AddHttpClient<ISearchEngineService, SearchEngineService>(c =>{c.BaseAddress = new Uri(googleLocation);}).AddHttpMessageHandler<LogHandler>().AddHttpMessageHandler<RetryHandler>();
}

从上面的代码可以清楚地看到，LogHandler设置在RetryHandler之前。LogHandler是第一个处理程序，因此它处理在调用HttpClient时需要直接发生的事情。这是LogHandler实现：

public class LogHandler : DelegatingHandler
{private readonly ILogger<LogHandler> _logger;public LogHandler(ILogger<LogHandler> logger){_logger = logger;}protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken){var response = await base.SendAsync(request, cancellationToken);_logger.LogInformation("{response}", response);return response;}
}

从上面的代码可以清楚地看到，此处理程序实现仅在调用base方法之后记录来自Web请求的响应。此基本方法触发的内容由第二个处理程序设置：RetryHandler。如果服务器意外错误，此处理程序将重试。如果它直接成功或给出3次以上的服务器错误，则最后的结果计数并将返回。

public class RetryHandler : DelegatingHandler
{protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken){HttpResponseMessage result = null;for (int i = 0; i < 3; i++){result = await base.SendAsync(request, cancellationToken);if (result.StatusCode >= HttpStatusCode.InternalServerError){continue;}return result;}return result;}
}

如前所述，需要将这些处理程序管理的HttpClient注入到SearchEngineService实例中。这个类只有一个方法。该方法调用HttpClient实例，并返回内容的长度作为响应。

public class SearchEngineService : ISearchEngineService
{private readonly HttpClient _httpClient;public SearchEngineService(HttpClient httpClient){_httpClient = httpClient;}public async Task<int> GetNumberOfCharactersFromSearchQuery(string toSearchFor){var result = await _httpClient.GetAsync($"/search?q={toSearchFor}");var content = await result.Content.ReadAsStringAsync();return content.Length;}
}

SearchEngineService是控制器类的依赖，这个控制器类有一个get方法，它将方法调用的结果作为ActionResult返回。这是控制器类。

[Route("api/[controller]")]
[ApiController]
public class SearchEngineController : ControllerBase
{private readonly ISearchEngineService _searchEngineService;public SearchEngineController(ISearchEngineService searchEngineService){_searchEngineService = searchEngineService;}[HttpGet("{queryEntry}", Name = "GetNumberOfCharacters")]public async Task<ActionResult<int>> GetNumberOfCharacters(string queryEntry){var numberOfCharacters = await _searchEngineService.GetNumberOfCharactersFromSearchQuery(queryEntry);return Ok(numberOfCharacters);}
}

要编写一个集成测试此控制器，我们使用IntegrationFixture（NuGet包这里，文档在这里，文章有一些类似的代码在这里）。外部依赖关系已由模拟服务器代替，该模拟服务器在第一个请求之后返回内部服务器错误，而在第二个请求之后成功。对我们的控制器方法的调用已完成。这会触发对SearchEngineService的调用，从而调用HttpClient。如前所述，此类调用触发对LogHandler的调用，此后触发对RetryHandler的调用。由于第一个调用给出服务器错误，因此重试完成。RetryHandler不触发LogHandler（反之亦然）。因此，我们的应用程序仅记录一个响应，而实际上有两个响应（一个失败和一个成功）。这是我们的集成测试的代码：

[Fact]
public async Task TestDelegate()
{// arrangeawait using (var fixture = new Fixture<Startup>()){using (var searchEngineServer = fixture.FreezeServer("Google")){SetupUnStableServer(searchEngineServer, "Response");var controller = fixture.Create<SearchEngineController>();// actvar response = await controller.GetNumberOfCharacters("Hoi");// assert, externalvar externalResponseMessages = searchEngineServer.LogEntries.Select(l => l.ResponseMessage).ToList();Assert.Equal(2, externalResponseMessages.Count);Assert.Equal((int)HttpStatusCode.InternalServerError, externalResponseMessages.First().StatusCode);Assert.Equal((int)HttpStatusCode.OK, externalResponseMessages.Last().StatusCode);// assert, internalvar loggedResponse = fixture.LogSource.GetLoggedObjects<HttpResponseMessage>().ToList();Assert.Single(loggedResponse);var externalResponseContent = await loggedResponse.Single().Value.Content.ReadAsStringAsync();Assert.Equal("Response", externalResponseContent);Assert.Equal(HttpStatusCode.OK, loggedResponse.Single().Value.StatusCode);Assert.Equal(8, ((OkObjectResult)response.Result).Value);}}
}private void SetupUnStableServer(FluentMockServer fluentMockServer, string response)
{fluentMockServer.Given(Request.Create().UsingGet()).InScenario("UnstableServer").WillSetStateTo("FIRSTCALLDONE").RespondWith(Response.Create().WithBody(response, encoding: Encoding.UTF8).WithStatusCode(HttpStatusCode.InternalServerError));fluentMockServer.Given(Request.Create().UsingGet()).InScenario("UnstableServer").WhenStateIs("FIRSTCALLDONE").RespondWith(Response.Create().WithBody(response, encoding: Encoding.UTF8).WithStatusCode(HttpStatusCode.OK));
}

如果查看上面显示的代码，则会看到两个断言部分。在第一个断言部分中，我们验证外部（模拟）服务器的日志。由于第一个Web请求失败。我们希望执行第二个Web请求（带有第二个响应），因此应该有两个响应，这正是我们验证的结果。