如何在C＃xUnit测试用例中模拟构造函数？

Question

Hi I am working on C# application. I am writing unit test case for controller level. Below is my controller code.

 public IActionResult TriggerProductEventCheck([FromQuery(Name = "timeout-secs")] int timeoutS = 120)
    {
      int productEventsCount = 0;
      if (productService.ProcessingEnabled)
      {
        var cts = new CancellationTokenSource();
        cts.CancelAfter(timeoutS * 1000);
        lock (SyncObject)
        {
          this.consumerClient.Subscribe(this.productEventTopicName);
          while (!cts.IsCancellationRequested)
          {
            var productEvent = this.eventDispatcher.Consume();
            long kafkaOffSet = productEvent.Offset.Value;
            Product product = new Product(productEvent, log);
            if (product.Options == null)
            {
              break;
            }

            if (product != null)
            {
              productService.ProcessProductEvents(product, kafkaOffSet);
            }

            productEventsCount++;
          }
        }
      }

Below is my unit test case.

 public void ShouldReturnIfNoSQSEvents()
      {
        var productEventController = MockProvider.Target<ProductEventController>();
        productEventController.GetDependency<IProductEventService>().ProcessingEnabled.Returns(true);

        productEventController.GetDependency<IEventDispatcher>().Consume().Returns(new Confluent.Kafka.ConsumeResult<string, Avro.Generic.GenericRecord>());

        productEventController.GetDependency<IConsumerClient>().Subscribe("test");
        var productclass = Substitute.For<Product>();
        var response = productEventController.Target.TriggerProductEventCheck() as JsonResult;
        ((int)response.StatusCode).ShouldBe(200);
      }

Whenever I run above unit test case, Control goes inside Product product = new Product(productEvent, log); I want to mock this particular line. May I know how to handle this condition? Any help would be appreciated. Thanks

Answer 1

There are at least two ways.

The most advisable: add a second method that allows you to inject the desired elements. You can call one method from the other to reuse code. This second method would be used for testing. The other shuld be left as is.

Another option is to use a framework that allows to implement shims for testing. For example: - Moles , that "allows to replace any .NET method with a delegate". Here you can see an example that allows to mock DateTime constructor: Nikolai Tillmann: Moles - Replace any .NET method with a delegate . - "Microsot fakes": Isolating Code Under Test with Microsoft Fakes . - There are some commercial testing frameworks that also support shims.

However, you should use the first proposed solution, or change the desing of your software, for example using Dependency Injection, so that you can easily control de dependencies in your unit testing. You should leave the use of shims for cases where you don't have any other option, for example injecting changes in third party libraries for which you don't have the option to modify their code.

You can google ".NET shims testing" to learn more about this technique.