因为HttpClient使用SendAsync方法来执行所有的HTTP请求，你可以重写SendAsync方法并模拟HttpClient。

对于将HttpClient创建到接口的封装，如下所示

public interface IServiceHelper
{
    HttpClient GetClient();
}

然后在您的服务中使用上述接口进行依赖注入，示例如下

public class SampleService
{
    private readonly IServiceHelper serviceHelper;

    public SampleService(IServiceHelper serviceHelper)
    {
        this.serviceHelper = serviceHelper;
    }

    public async Task<HttpResponseMessage> Get(int dummyParam)
    {
        try
        {
            var dummyUrl = "http://www.dummyurl.com/api/controller/" + dummyParam;
            var client = serviceHelper.GetClient();
            HttpResponseMessage response = await client.GetAsync(dummyUrl);               

            return response;
        }
        catch (Exception)
        {
            // log.
            throw;
        }
    }
}

Now in unit test project create a helper class for mocking SendAsync. Here it is a FakeHttpResponseHandler class which is inheriting DelegatingHandler which will provide an option to override the SendAsync method. After overriding the SendAsync method need to setup a response for each HTTP Request which is calling SendAsync method, for that create a Dictionary with key as Uri and value as HttpResponseMessage so that whenever there is a HTTP Request and if the Uri matches SendAsync will return the configured HttpResponseMessage.

public class FakeHttpResponseHandler : DelegatingHandler
{
    private readonly IDictionary<Uri, HttpResponseMessage> fakeServiceResponse;
    private readonly JavaScriptSerializer javaScriptSerializer;
    public FakeHttpResponseHandler()
    {
        fakeServiceResponse =  new Dictionary<Uri, HttpResponseMessage>();
        javaScriptSerializer =  new JavaScriptSerializer();
    }

    /// <summary>
    /// Used for adding fake httpResponseMessage for the httpClient operation.
    /// </summary>
    /// <typeparam name="TQueryStringParameter"> query string parameter </typeparam>
    /// <param name="uri">Service end point URL.</param>
    /// <param name="httpResponseMessage"> Response expected when the service called.</param>
    public void AddFakeServiceResponse(Uri uri, HttpResponseMessage httpResponseMessage)
    {
        fakeServiceResponse.Remove(uri);
        fakeServiceResponse.Add(uri, httpResponseMessage);
    }

    /// <summary>
    /// Used for adding fake httpResponseMessage for the httpClient operation having query string parameter.
    /// </summary>
    /// <typeparam name="TQueryStringParameter"> query string parameter </typeparam>
    /// <param name="uri">Service end point URL.</param>
    /// <param name="httpResponseMessage"> Response expected when the service called.</param>
    /// <param name="requestParameter">Query string parameter.</param>
    public void AddFakeServiceResponse<TQueryStringParameter>(Uri uri, HttpResponseMessage httpResponseMessage, TQueryStringParameter requestParameter)
    {
        var serilizedQueryStringParameter = javaScriptSerializer.Serialize(requestParameter);
        var actualUri = new Uri(string.Concat(uri, serilizedQueryStringParameter));
        fakeServiceResponse.Remove(actualUri);
        fakeServiceResponse.Add(actualUri, httpResponseMessage);
    }

    // all method in HttpClient call use SendAsync method internally so we are overriding that method here.
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        if(fakeServiceResponse.ContainsKey(request.RequestUri))
        {
            return Task.FromResult(fakeServiceResponse[request.RequestUri]);
        }

        return Task.FromResult(new HttpResponseMessage(HttpStatusCode.NotFound)
        {
            RequestMessage = request,
            Content = new StringContent("Not matching fake found")
        });
    }
}

通过模仿框架或如下所示创建IServiceHelper的新实现。 我们可以使用这个FakeServiceHelper类来注入FakeHttpResponseHandler类，这样每当这个类创建HttpClient时，它都会使用FakeHttpResponseHandler类而不是实际的实现。

public class FakeServiceHelper : IServiceHelper
{
    private readonly DelegatingHandler delegatingHandler;

    public FakeServiceHelper(DelegatingHandler delegatingHandler)
    {
        this.delegatingHandler = delegatingHandler;
    }

    public HttpClient GetClient()
    {
        return new HttpClient(delegatingHandler);
    }
}

And in test configure FakeHttpResponseHandler class by adding the Uri and expected HttpResponseMessage. The Uri should be the actual serviceendpoint Uri so that when the overridden SendAsync method is called from actual service implementation it will match the Uri in Dictionary and respond with the configured HttpResponseMessage. After configuring inject the FakeHttpResponseHandler object to the fake IServiceHelper implementation. Then inject the FakeServiceHelper class to the actual service which will make the actual service to use the override SendAsync method.

[TestClass]
public class SampleServiceTest
{
    private FakeHttpResponseHandler fakeHttpResponseHandler;

    [TestInitialize]
    public void Initialize()
    {
        fakeHttpResponseHandler = new FakeHttpResponseHandler();
    }

    [TestMethod]
    public async Task GetMethodShouldReturnFakeResponse()
    {
        Uri uri = new Uri("http://www.dummyurl.com/api/controller/");
        const int dummyParam = 123456;
        const string expectdBody = "Expected Response";

        var expectedHttpResponseMessage = new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = new StringContent(expectdBody)
        };

        fakeHttpResponseHandler.AddFakeServiceResponse(uri, expectedHttpResponseMessage, dummyParam);

        var fakeServiceHelper = new FakeServiceHelper(fakeHttpResponseHandler);

        var sut = new SampleService(fakeServiceHelper);

        var response = await sut.Get(dummyParam);

        var responseBody = await response.Content.ReadAsStringAsync();

        Assert.AreEqual(HttpStatusCode.OK, response.StatusCode);
        Assert.AreEqual(expectdBody, responseBody);
    }
}

GitHub链接:有示例实现

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(...);
    }
}

HttpClient的可扩展性在于传递给构造函数的HttpMessageHandler。它的目的是允许特定于平台的实现，但您也可以模拟它。不需要为HttpClient创建装饰器包装。

如果你更喜欢DSL而不是使用Moq，我在GitHub/Nuget上有一个库，它可以让事情变得更容易一些:https://github.com/richardszalay/mockhttp

Nuget包RichardSzalay。MockHttp可以在这里找到。

var mockHttp = new MockHttpMessageHandler();

// Setup a respond for the user api (including a wildcard in the URL)
mockHttp.When("http://localhost/api/user/*")
        .Respond("application/json", "{'name' : 'Test McGee'}"); // Respond with JSON

// Inject the handler or client into your application code
var client = new HttpClient(mockHttp);

var response = await client.GetAsync("http://localhost/api/user/1234");
// or without async: var response = client.GetAsync("http://localhost/api/user/1234").Result;

var json = await response.Content.ReadAsStringAsync();

// No network connection required
Console.Write(json); // {'name' : 'Test McGee'}

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();  
}

我做了一些非常简单的事情，因为我在一个依赖注入环境中。

public class HttpHelper : IHttpHelper
{
    private ILogHelper _logHelper;

    public HttpHelper(ILogHelper logHelper)
    {
        _logHelper = logHelper;
    }

    public virtual async Task<HttpResponseMessage> GetAsync(string uri, Dictionary<string, string> headers = null)
    {
        HttpResponseMessage response;
        using (var client = new HttpClient())
        {
            if (headers != null)
            {
                foreach (var h in headers)
                {
                    client.DefaultRequestHeaders.Add(h.Key, h.Value);
                }
            }
            response = await client.GetAsync(uri);
        }

        return response;
    }

    public async Task<T> GetAsync<T>(string uri, Dictionary<string, string> headers = null)
    {
        ...

        rawResponse = await GetAsync(uri, headers);

        ...
    }

}

这个笑话是:

    [TestInitialize]
    public void Initialize()
    {
       ...
        _httpHelper = new Mock<HttpHelper>(_logHelper.Object) { CallBase = true };
       ...
    }

    [TestMethod]
    public async Task SuccessStatusCode_WithAuthHeader()
    {
        ...

        _httpHelper.Setup(m => m.GetAsync(_uri, myHeaders)).Returns(
            Task<HttpResponseMessage>.Factory.StartNew(() =>
            {
                return new HttpResponseMessage(System.Net.HttpStatusCode.OK)
                {
                    Content = new StringContent(JsonConvert.SerializeObject(_testData))
                };
            })
        );
        var result = await _httpHelper.Object.GetAsync<TestDTO>(...);

        Assert.AreEqual(...);
    }

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);
    }
}