使用模拟的IHttpFilter和IHttpContent对HttpClient进行单元测试，MockedHttpFilter抛出System.InvalidCastException

Question

我有一个依赖于Windows.Web.Http (Windows10UAP App)的HttpClient的类。我想进行单元测试，因此我需要“模拟”HttpClient来设置Get-Call应该返回什么。我从一个使用手写模拟的IHttpFilter和IHttpContent的HttpClient的“简单”单元测试开始。它没有像预期的那样工作，并且我在测试资源管理器中得到了一个InvalidCastException。

单元测试看起来像这样：

    [TestMethod]
    public async Task TestMockedHttpFilter()
    {
        MockedHttpContent mockedContent = new MockedHttpContent("Content from MockedHttpContent");
        MockedHttpFilter mockedHttpFilter = new MockedHttpFilter(HttpStatusCode.Ok, mockedContent);

        HttpClient httpClient = new HttpClient(mockedHttpFilter);
        var resultContentTask = await httpClient.SendRequestAsync(new HttpRequestMessage(HttpMethod.Get, new Uri("http://dontcare.ch"))).AsTask().ConfigureAwait(false);
        // Test stops here, throwing System.InvalidCastException: Specified cast is not valid

        // Code not reached...
        var result = await resultContentTask.Content.ReadAsStringAsync();
        Assert.AreEqual("Content from MockedHttpContent", result);
    }

我在MockedHttpFilter中实现了IHttpFilter：

public class MockedHttpFilter : IHttpFilter
{
    private HttpStatusCode _statusCode;
    private IHttpContent _content;

    public MockedHttpFilter(HttpStatusCode statusCode, IHttpContent content)
    {
        _statusCode = statusCode;
        _content = content;
    }

    public IAsyncOperationWithProgress<HttpResponseMessage, HttpProgress> SendRequestAsync(HttpRequestMessage request)
    {
        return AsyncInfo.Run<HttpResponseMessage, HttpProgress>((token, progress) =>
        Task.Run<HttpResponseMessage>(()=>
        {
            HttpResponseMessage response = new HttpResponseMessage(_statusCode);
            response.Content = _content;
            return response; // Exception thrown after return, but not catched by code/debugger...
        }));
    }
}

我在MockedHttpContent中实现了IHttpContent：

public class MockedHttpContent : IHttpContent
{
    private string _contentToReturn;

    public MockedHttpContent(string contentToReturn)
    {
        _contentToReturn = contentToReturn;
    }

    public HttpContentHeaderCollection Headers
    {
        get
        {
            return new HttpContentHeaderCollection();
        }
    }

    public IAsyncOperationWithProgress<string, ulong> ReadAsStringAsync()
    {
        return AsyncInfo.Run<string, ulong>((token, progress) => Task.Run<string>(() =>
        {
            return _contentToReturn;
        }));
    }
}

Test-Explorer结果视图中的错误：

Test Name:  TestMockedHttpFilter
Test FullName:  xxx.UnitTests.xxxHttpClientUnitTests.TestMockedHttpFilter
Test Source:    xxx.UnitTests\xxxHttpClientUnitTests.cs : line 22
Test Outcome:   Failed
Test Duration:  0:00:00.1990313

Result StackTrace:  
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at xxx.UnitTests.xxxHttpClientUnitTests.<TestMockedHttpFilter>d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Result Message: Test method xxx.UnitTests.xxxHttpClientUnitTests.TestMockedHttpFilter threw exception: 
System.InvalidCastException: Specified cast is not valid.

首先，不确定为什么会抛出异常/我做错了什么。也许有人可以给我指明正确的方向，或者给出一个提示，接下来要检查/测试什么？

其次，有没有更好的方法来使用HttpClient依赖(Windows10UAP)对代码进行单元测试？

Answer 1

您的单元测试没有任何意义，您实际上并不是在测试您的类，而是在尝试测试HttpClient类。我将假设一些事情，因为我们没有在您的问题中测试的方法；我假设您做了类似以下的事情：

public async Task<MyCustomClass> MethodUnderTest()
{
    // ...

    using (var client = new HttpClient(...))
    {
        // ...

        var json = response.Content.ReadAsStringAsync();
        return JsonConvert.Deserialize<MyCustomClass>(json);
    }
}

如果是这种情况，那么您必须了解您的类不接受某些依赖项。您可以更改类，以便注入每个外部依赖项，但这可能有点夸张……您真的希望使用和实例化您的类的任何人都必须向您提供HttpClient吗？因此，更好的选择是使用模拟框架，它可以模拟具体类的依赖关系；大多数模拟框架可以处理模拟接口(这些接口很容易)，但很少有框架可以处理模拟具体类。

我建议使用的框架是Microsoft Fakes框架。Microsoft Fakes支持填充(隔离)和存根(Mock)。隔离是控制具体类的成员调用所需要的。

那么，根据我的例子，哪些成员需要控制？

1. HttpResponseMessage.Content_Get
2. HttpContent.ReadAsStringAsync

我不认为您需要改变JsonConvert.Deserialize<T>()成员的行为，但如果您愿意的话，您可以这样做。Microsoft Fakes框架一开始有点让人望而生畏，但一旦你开始使用它并让它工作，它就会变得更容易使用。或者，您可以使用其他支持隔离的框架：

来自Telerik

• TypeMock
• JustMock的隔离器

也许其他人存在，但我对他们不熟悉。

Answer 2

您似乎没有使用与httpClient.SendRequestAsync(...)的返回值匹配的AsTask重载

尝试更改此设置

var resultContentTask = await httpClient.SendRequestAsync(new HttpRequestMessage(HttpMethod.Get, new Uri("http://dontcare.ch"))).AsTask().ConfigureAwait(false);

到这个

var resultContentTask = await httpClient.SendRequestAsync(new HttpRequestMessage(HttpMethod.Get, new Uri("http://dontcare.ch"))).AsTask<IAsyncOperationWithProgress<HttpResponseMessage, HttpProgress>>().ConfigureAwait(false);

Answer 3

最简单的解决方案是使用这个库来模拟HttpClient

https://github.com/richardszalay/mockhttp

PM> Install-Package RichardSzalay.MockHttp

我还用你的附加代码测试了它

       //Arrange
        var mockHttp = new MockHttpMessageHandler();
        mockHttp.When("http://dontcare.ch")
            .Respond("application/txt", "Content from MockedHttpContent"); // Respond with content
        var client = mockHttp.ToHttpClient();

        //Act
        var response = await client.GetAsync("http://dontcare.ch");
        var result = await response.Content.ReadAsStringAsync();

        //Assert
        Assert.AreEqual("Content from MockedHttpContent", result);