.NET & Nsubstitute 模拟Http请求

我们的代码中有时候会需要调用其他平台的接口，在做单元测试的时候，我们不需要测试这些第三方接口是否生效，接口是否有问题，但是如果我们直接调用第三方接口，这些接口的错误又会影响我们正常的测试工作，所以我们应该怎么办呢？我们可以模拟这些接口，让其返回我们期待的返回结果，这样我们就可以顺利的进行内部代码的测试了。
这里我们继承HttpMessageHandler，写一个MockHttpMessageHandler。

public class MockHttpMessageHandler : HttpMessageHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return Task.FromResult(MockSend(request, cancellationToken));
    }

    protected override HttpResponseMessage Send(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return MockSend(request, cancellationToken);
    }

    public virtual HttpResponseMessage MockSend(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        throw new NotImplementedException();
    }
}

然后我们通过Substitute.ForPartsOf<T>()方法创建一个MockHttpMessageHandler对象，将其传参给HttpClient对象。

var mockHttpMessageHandler = Substitute.ForPartsOf<MockHttpMessageHandler>();
httpClient = new HttpClient(mockHttpMessageHandler);

然后通过Nsubstitute的Return等方法设置返回结果HttpResponseMessage。

HttpResponseMessage mockResponse = new HttpResponseMessage(HttpStatusCode.OK);
mockResponse.Content = new StringContent(returnMessage);
mockHttpMessageHandler.MockSend(Arg.Any<HttpRequestMessage>(), Arg.Any<CancellationToken>())
    .ReturnsForAnyArgs(mockResponse);

这里我们设置任意参数的MockSend方法均返回HttpStatusCode=OK，返回值为returnMessage。
在实际测试过程中，我们可以根据具体的情况返回不同的结果。
整段测试代码如下：

public class MockHttpClientUnitTest
{
    HttpClient httpClient;
    const string returnMessage = "Ok";
    public MockHttpClientUnitTest()
    {
        var mockHttpMessageHandler = Substitute.ForPartsOf<MockHttpMessageHandler>();
        httpClient = new HttpClient(mockHttpMessageHandler);
        //mock response
        HttpResponseMessage mockResponse = new HttpResponseMessage(HttpStatusCode.OK);
        mockResponse.Content = new StringContent(returnMessage);
        mockHttpMessageHandler.MockSend(Arg.Any<HttpRequestMessage>(), Arg.Any<CancellationToken>())
            .ReturnsForAnyArgs(mockResponse);
    }

    [Fact]
    public async void Test()
    {
        (await (await httpClient.GetAsync("https://cn.bing.com/")).Content.ReadAsStringAsync()).ShouldBe(returnMessage);
    }
}

示例代码

MockHttpClientUnitTest
MockHttpMessageHandler

参考资料

Partial subs and test spies
How to Mock HttpClient using NSubstitute