单元测试Windows.Web.Http HttpClient与模拟的IHttpFilter和IHttpContent，MockedHttpFilter抛出System.InvalidCastExce

Posted 2021-04-13

我有一个类依赖于Windows.Web.Http（Windows 10 UAP App）中的HttpClient。我想进行单元测试，因此我需要“模拟”HttpClient来设置Get-Call应该返回的内容。我开始使用HttpClient进行“简单”的单元测试，使用手写模拟的IHttpFilter和IHttpContent。它没有按预期工作，我在Test-Explorer中得到一个InvalidCastException。

单元测试看起来像：

    [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.UnitTestsxxxHttpClientUnitTests.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，不确定抛出异常的原因/我做错了什么。也许有人可以指出我正确的方向或提示下一步检查/测试什么？

其次，是否有更好的方法来使用HttpClient依赖（Windows 10 UAP）对代码进行单元测试？

答案

你的单元测试没有意义，你并没有真正测试你的类，而是你试图测试HttpClient类。我会假设一些事情，因为我们在你的问题中没有测试方法;我假设您执行以下操作：

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，但这可能是一个轻微的矫枉过正......您真的希望任何消耗和实例化您的类的人必须向您提供HttpClient吗？所以，更好的选择是使用一个模拟框架，它可以模拟一个具体类的依赖;大多数模拟框架都可以处理模拟接口（这些很容易），但很少有人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Shims（隔离）和Stubs（Mocks）。隔离是您控制具体类的成员调用所需的。

那么，根据我的例子，需要控制哪些成员？

1. HttpResponseMessage.Content_Get
2. HttpContent.ReadAsStringAsync

我认为你不需要改变JsonConvert.Deserialize<T>()成员的行为，但如果你愿意，你可以。微软Fakes框架起初有点令人生畏，但是一旦你开始使用它并使其工作，它将变得更容易使用。或者，您可以使用其他支持隔离的框架：

• 来自Telerik的JustMock
• TypeMock隔离器

也许其他人存在，我不熟悉他们。

另一答案

看来你没有使用与AsTask的返回值匹配的httpClient.SendRequestAsync(...)重载

尝试改变这一点

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

对此

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

另一答案

最简单的解决方案是使用此库来模拟HttpClient

https://github.com/richardszalay/mockhttp

PM> Install-Package RichardSzalay.MockHttp

我还用附加的代码测试了它

       //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);