如何使用 .NET Core 3 Web API 和 Moq 模拟 ExceptionContext 进行测试

Question

为了在 .NET Core 3 中正确测试自定义 ExceptionFilterAttribute，我们需要使用 Moq 在 Xunit 测试中模拟 ExceptionContext。我怎样才能做到这一点？

异常过滤器：

public class CustomExceptionFilter : ExceptionFilterAttribute
{
  public override OnException(ExceptionContext context)
  {
    /* Code here that handles exceptions */
  }
}

我到处都找过了，但找不到一种好方法来模拟这些东西，以确保不会因其他缺失的依赖项而引发异常。我怎样才能ExceptionContext轻松嘲笑？

Answer 1

问题在于，如果您尝试模拟ExceptionContext以返回不同的异常类型，那么您实际上想要模拟异常，然后在实例化ExceptionContext.

首先，要ExceptionContext在测试中实例化 for 过滤器，您需要能够实例化ActionContext. 这ActionContext与我们的测试无关，但依赖关系树需要它，因此我们必须以尽可能少的定制来实例化它：

var actionContext = new ActionContext()
{
  HttpContext = new DefaultHttpContext(),
  RouteData = new RouteData(),
  ActionDescriptor = new ActionDescriptor()
};

在能够实例化之后，ActionContext您可以实例化 ExceptionContext。在这里，我们还模拟了用于构建ExceptionContext. 这是最重要的一步，因为这是改变我们正在测试的行为的值。

// The stacktrace message and source member variables are virtual and so we can stub them here.
var mockException = new Mock<Exception>();

mockException.Setup(e => e.StackTrace)
  .Returns("Test stacktrace");
mockException.Setup(e => e.Message)
  .Returns("Test message");
mockException.Setup(e => e.Source)
  .Returns("Test source");

var exceptionContext = new ExceptionContext(actionContext, new List<FilterMetadata>())
{
  Exception = mockException.Object
};

通过这种方式，您可以在给定不同异常类型时充分测试异常过滤器的行为。

完整代码：

[Fact]
public void TestExceptionFilter()
{
  var actionContext = new ActionContext()
  {
    HttpContext = new DefaultHttpContext(),
    RouteData = new RouteData(),
    ActionDescriptor = new ActionDescriptor()
  };

  // The stacktrace message and source member variables are virtual and so we can stub them here.
  var mockException = new Mock<Exception>();

  mockException.Setup(e => e.StackTrace)
    .Returns("Test stacktrace");
  mockException.Setup(e => e.Message)
    .Returns("Test message");
  mockException.Setup(e => e.Source)
    .Returns("Test source");

  // the List<FilterMetadata> here doesn't have much relevance in the test but is required 
  // for instantiation. So we instantiate a new instance of it with no members to ensure
  // it does not effect the test.
  var exceptionContext = new ExceptionContext(actionContext, new List<FilterMetadata>())
  {
    Exception = mockException.Object
  };

  var filter = new CustomExceptionFilter();

  filter.OnException(exceptionContext);

  // Assumption here that your exception filter modifies status codes.
  // Just used as an example of how you can assert in this test.
  context.HttpContext.Response.StatusCode.Should().Be(500, 
    "Because the response code should match the exception thrown.");
}