如何使用自定义HttpClientHandler配置对HttpClient进行单元测试/依赖注入类

Question

我正在寻找有关如何改进当前测试类（以下示例）的设计的建议，该类依赖于具有自定义HttpClient配置的HttpClientHandler。我通常使用构造函数注入来注入一个在整个应用程序中一致的HttpClient，但是，因为这是在类库中，所以我不能依赖库的使用者来正确设置HttpClientHandler。

null

这个设置对我来说似乎相当复杂，我希望有人能提出一个改进，但我知道这可能是相当主观的，所以如果有任何既定的模式或设计规则要遵循在这种情况下，我会非常感谢听到他们。

我包含了DownloadSomethingAsync()方法，只是为了演示为什么HttpClientHandler需要非标准配置。默认情况下，重定向响应在内部自动重定向而不返回响应，我需要重定向响应，这样我就可以将它包装在一个报告下载进度的类中（其功能与此问题无关）。

public class DemoClass
{
    private static readonly HttpClient defaultHttpClient = new HttpClient(
            new HttpClientHandler
            {
                AllowAutoRedirect = false
            });

    private readonly ILogger<DemoClass> logger;
    private readonly HttpClient httpClient;

    public DemoClass(ILogger<DemoClass> logger) : this(logger, defaultHttpClient) { }

    private DemoClass(ILogger<DemoClass> logger, HttpClient httpClient)
    {
        this.logger = logger ?? throw new ArgumentNullException(nameof(logger));
        this.httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
    }

    [Obsolete("This is only provided for testing and should not be used in calling code")]
    internal static DemoClass CreateWithCustomHttpClient(ILogger<DemoClass> logger, HttpClient httpClient)
        => new DemoClass(logger, httpClient);

    public async Task<FileSystemInfo> DownloadSomethingAsync(CancellationToken ct = default)
    {
        // Build the request
        logger.LogInformation("Sending request for download");
        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, "http://example.com/downloadredirect");

        // Send the request
        HttpResponseMessage response = await httpClient.SendAsync(request, ct);

        // Analyse the result
        switch (response.StatusCode)
        {
            case HttpStatusCode.Redirect:
                break;
            case HttpStatusCode.NoContent:
                return null;
            default: throw new InvalidOperationException();
        }

        // Get the redirect location
        Uri redirect = response.Headers.Location;

        if (redirect == null)
            throw new InvalidOperationException("Redirect response did not contain a redirect URI");

        // Create a class to handle the download with progress tracking
        logger.LogDebug("Wrapping release download request");
        IDownloadController controller = new HttpDownloadController(redirect);

        // Begin the download
        logger.LogDebug("Beginning release download");

        return await controller.DownloadAsync();
    }
}

Answer 1

在我看来，我会在Microsoft.Extensions.http中使用IHttpClientFactory，并创建一个自定义依赖项注入扩展，供类库的使用者使用：

public static class DemoClassServiceCollectionExtensions
{
    public static IServiceCollection AddDemoClass(
        this IServiceCollection services, 
        Func<HttpMessageHandler> configureHandler = null)
    {
        // Configure named HTTP client with primary message handler
        var builder= services.AddHttpClient(nameof(DemoClass));

        if (configureHandler == null)
        {
            builder = builder.ConfigurePrimaryHttpMessageHandler(
                () => new HttpClientHandler
                {
                    AllowAutoRedirect = false
                });
        }
        else
        {
            builder = builder.ConfigurePrimaryHttpMessageHandler(configureHandler);
        }

        services.AddTransient<DemoClass>();

        return services;
    }
}

在DemoClass中，使用IHttpClientFactory创建命名HTTP客户端：

class DemoClass
{
    private readonly HttpClient _client;

    public DemoClass(IHttpClientFactory httpClientFactory)
    {
        // This named client will have pre-configured message handler
        _client = httpClientFactory.CreateClient(nameof(DemoClass));
    }

    public async Task DownloadSomethingAsync()
    {
        // omitted
    }
}

您可以要求使用者必须调用AddDemoClass才能使用DemoClass:

var services = new ServiceCollection();
services.AddDemoClass();

通过这种方式，您可以隐藏HTTP客户端构造的详细信息。

同时，在测试中，您可以模拟IHttpClientFactory以返回HttpClient用于测试目的。