基于其他答案，我建议这样的代码，它没有任何外部依赖:

[TestClass]
public class MyTestClass
{
    [TestMethod]
    public async Task MyTestMethod()
    {
        var httpClient = new HttpClient(new MockHttpMessageHandler());

        var content = await httpClient.GetStringAsync("http://some.fake.url");

        Assert.AreEqual("Content as string", content);
    }
}

public class MockHttpMessageHandler : HttpMessageHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        var responseMessage = new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = new StringContent("Content as string")
        };

        return await Task.FromResult(responseMessage);
    }
}

受PointZeroTwo答案的启发，下面是一个使用NUnit和FakeItEasy的示例。

本例中的SystemUnderTest是您想要测试的类-没有为它提供示例内容，但我假设您已经有了它!

[TestFixture]
public class HttpClientTests
{
    private ISystemUnderTest _systemUnderTest;
    private HttpMessageHandler _mockMessageHandler;

    [SetUp]
    public void Setup()
    {
        _mockMessageHandler = A.Fake<HttpMessageHandler>();
        var httpClient = new HttpClient(_mockMessageHandler);

        _systemUnderTest = new SystemUnderTest(httpClient);
    }

    [Test]
    public void HttpError()
    {
        // Arrange
        A.CallTo(_mockMessageHandler)
            .Where(x => x.Method.Name == "SendAsync")
            .WithReturnType<Task<HttpResponseMessage>>()
            .Returns(Task.FromResult(new HttpResponseMessage
            {
                StatusCode = HttpStatusCode.InternalServerError,
                Content = new StringContent("abcd")
            }));

        // Act
        var result = _systemUnderTest.DoSomething();

        // Assert
        // Assert.AreEqual(...);
    }
}

正如注释中提到的，您需要抽象HttpClient，这样就不会与它耦合。我以前也做过类似的事情。我会试着把我的方法和你们要做的方法相适应。

首先看一下HttpClient类，并决定需要它提供哪些功能。

以下是一种可能性:

public interface IHttpClient {
    System.Threading.Tasks.Task<T> DeleteAsync<T>(string uri) where T : class;
    System.Threading.Tasks.Task<T> DeleteAsync<T>(Uri uri) where T : class;
    System.Threading.Tasks.Task<T> GetAsync<T>(string uri) where T : class;
    System.Threading.Tasks.Task<T> GetAsync<T>(Uri uri) where T : class;
    System.Threading.Tasks.Task<T> PostAsync<T>(string uri, object package);
    System.Threading.Tasks.Task<T> PostAsync<T>(Uri uri, object package);
    System.Threading.Tasks.Task<T> PutAsync<T>(string uri, object package);
    System.Threading.Tasks.Task<T> PutAsync<T>(Uri uri, object package);
}

如前所述，这是为了特定目的。我完全抽象出了对HttpClient的依赖，并专注于我想要返回的东西。您应该评估如何抽象HttpClient以只提供您想要的必要功能。

这将允许您只模拟需要测试的内容。

我甚至建议完全放弃IHttpHandler，而使用HttpClient抽象IHttpClient。但我只是没有选择，因为你可以用抽象客户端的成员替换你的处理程序接口的主体。

IHttpClient的实现可以用来包装/改编一个真正的/具体的HttpClient或任何其他对象，它可以用来发出HTTP请求，因为你真正想要的是一个服务，它提供了与HttpClient相反的功能。使用抽象是一种干净(我的观点)和可靠的方法，并且可以使您的代码更易于维护，如果您需要在框架更改时将底层客户端切换为其他东西。

下面是如何实现的代码片段。

/// <summary>
/// HTTP Client adaptor wraps a <see cref="System.Net.Http.HttpClient"/> 
/// that contains a reference to <see cref="ConfigurableMessageHandler"/>
/// </summary>
public sealed class HttpClientAdaptor : IHttpClient {
    HttpClient httpClient;

    public HttpClientAdaptor(IHttpClientFactory httpClientFactory) {
        httpClient = httpClientFactory.CreateHttpClient(**Custom configurations**);
    }

    //...other code

     /// <summary>
    ///  Send a GET request to the specified Uri as an asynchronous operation.
    /// </summary>
    /// <typeparam name="T">Response type</typeparam>
    /// <param name="uri">The Uri the request is sent to</param>
    /// <returns></returns>
    public async System.Threading.Tasks.Task<T> GetAsync<T>(Uri uri) where T : class {
        var result = default(T);
        //Try to get content as T
        try {
            //send request and get the response
            var response = await httpClient.GetAsync(uri).ConfigureAwait(false);
            //if there is content in response to deserialize
            if (response.Content.Headers.ContentLength.GetValueOrDefault() > 0) {
                //get the content
                string responseBodyAsText = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
                //desrialize it
                result = deserializeJsonToObject<T>(responseBodyAsText);
            }
        } catch (Exception ex) {
            Log.Error(ex);
        }
        return result;
    }

    //...other code
}

正如你在上面的例子中所看到的，很多通常与使用HttpClient相关的繁重工作都隐藏在抽象后面。

然后可以用抽象客户端注入连接类

public class Connection
{
    private IHttpClient _httpClient;

    public Connection(IHttpClient httpClient)
    {
        _httpClient = httpClient;
    }
}

然后，您的测试可以模拟SUT所需的内容

private IHttpClient _httpClient;

[TestMethod]
public void TestMockConnection()
{
    SomeModelObject model = new SomeModelObject();
    var httpClientMock = new Mock<IHttpClient>();
    httpClientMock.Setup(c => c.GetAsync<SomeModelObject>(It.IsAny<string>()))
        .Returns(() => Task.FromResult(model));

    _httpClient = httpClientMock.Object;

    var client = new Connection(_httpClient);

    // Assuming doSomething uses the client to make
    // a request for a model of type SomeModelObject
    client.doSomething();  
}

经过仔细搜索，我找到了实现这一目标的最佳方法。

    private HttpResponseMessage response;

    [SetUp]
    public void Setup()
    {
        var handlerMock = new Mock<HttpMessageHandler>();

        handlerMock
           .Protected()
           .Setup<Task<HttpResponseMessage>>(
              "SendAsync",
              ItExpr.IsAny<HttpRequestMessage>(),
              ItExpr.IsAny<CancellationToken>())
           // This line will let you to change the response in each test method
           .ReturnsAsync(() => response);

        _httpClient = new HttpClient(handlerMock.Object);

        yourClinet = new YourClient( _httpClient);
    }

正如你所注意到的，我使用了Moq和Moq。保护包。

您的接口公开了具体的HttpClient类，因此任何使用该接口的类都被绑定到它，这意味着它不能被模拟。

HttpClient不继承任何接口，所以你必须自己编写。我建议使用类似装饰器的图案:

public interface IHttpHandler
{
    HttpResponseMessage Get(string url);
    HttpResponseMessage Post(string url, HttpContent content);
    Task<HttpResponseMessage> GetAsync(string url);
    Task<HttpResponseMessage> PostAsync(string url, HttpContent content);
}

你的类看起来是这样的:

public class HttpClientHandler : IHttpHandler
{
    private HttpClient _client = new HttpClient();

    public HttpResponseMessage Get(string url)
    {
        return GetAsync(url).Result;
    }

    public HttpResponseMessage Post(string url, HttpContent content)
    {
        return PostAsync(url, content).Result;
    }

    public async Task<HttpResponseMessage> GetAsync(string url)
    {
        return await _client.GetAsync(url);
    }

    public async Task<HttpResponseMessage> PostAsync(string url, HttpContent content)
    {
        return await _client.PostAsync(url, content);
    }
}

所有这些的重点是HttpClientHandler创建了它自己的HttpClient，然后你当然可以创建多个以不同方式实现IHttpHandler的类。

这种方法的主要问题是，你实际上是在编写一个只调用另一个类中的方法的类，然而你可以创建一个从HttpClient继承的类(参见Nkosi的例子，这是一个比我的方法更好的方法)。如果HttpClient有一个可以模拟的接口，事情会简单得多，不幸的是它没有。

This example is not the golden ticket however. IHttpHandler still relies on HttpResponseMessage, which belongs to System.Net.Http namespace, therefore if you do need other implementations other than HttpClient, you will have to perform some kind of mapping to convert their responses into HttpResponseMessage objects. This of course is only a problem if you need to use multiple implementations of IHttpHandler but it doesn't look like you do so it's not the end of the world, but it's something to think about.

无论如何，您可以简单地模拟IHttpHandler，而不必担心具体的HttpClient类，因为它已经被抽象掉了。

我建议测试非异步方法，因为这些方法仍然调用异步方法，但不必担心单元测试异步方法的麻烦，参见这里

加入这个派对有点晚了，但我喜欢在带有下游REST依赖的dotnet核心微服务的集成测试中尽可能使用wiremocking (https://github.com/WireMock-Net/WireMock.Net)。

通过实现一个扩展IHttpClientFactory的TestHttpClientFactory，我们可以重写这个方法

HttpClient CreateClient(字符串名称)

所以当你在应用中使用命名客户端时，你可以控制返回一个连接到你的wiremock的HttpClient。

这种方法的好处是，您不会更改正在测试的应用程序中的任何内容，并且允许课程集成测试对您的服务执行实际的REST请求，并模拟实际下游请求应该返回的json(或任何东西)。这将导致在应用程序中进行简洁的测试和尽可能少的模拟。

    public class TestHttpClientFactory : IHttpClientFactory 
{
    public HttpClient CreateClient(string name)
    {
        var httpClient = new HttpClient
        {
            BaseAddress = new Uri(G.Config.Get<string>($"App:Endpoints:{name}"))
            // G.Config is our singleton config access, so the endpoint 
            // to the running wiremock is used in the test
        };
        return httpClient;
    }
}

and

// in bootstrap of your Microservice
IHttpClientFactory factory = new TestHttpClientFactory();
container.Register<IHttpClientFactory>(factory);