Unit Testing error “Object reference not set to an instance of an object.”

问题

In my controller I want to test if the controller is calling the repository method. Here is the method in controller

[HttpGet]
public ActionResult GetModulePropertyName(string moduleTypeValue)
{
  var temp = _modulerepository.GetModuleKindPropertyNames(moduleTypeValue);

  IList<Property> model = temp
     .Select(item => new Property {Name = item})
     .ToList();

  return PartialView("GetModulePropertyName",model);
}

And here is the test method

[TestMethod]
public void GetModulePropertyName_Action_Calls_GetModuleKindPropertyNames()
{
  _mockRepository.Stub(x => x.GetModuleKindPropertyNames(Arg<string>.Is.Anything));

  _controller.GetModulePropertyName(Arg<string>.Is.Anything);

  _mockRepository.AssertWasCalled(x=>x.GetModuleKindPropertyNames(Arg<string>.Is.Anything));
}

It throws an error saying

Test method AdminPortal.Tests.ModuleControllerTests.GetModulePropertyName_Action_Calls_GetModuleKindPropertyNames threw exception: 
System.NullReferenceException: Object reference not set to an instance of an object.
    at System.Linq.Queryable.Select(IQueryable`1 source, Expression`1 selector)
   at AdminPortal.Areas.Hardware.Controllers.ModuleController.GetModulePropertyName(String moduleTypeValue) in ModuleController.cs: line 83
   at AdminPortal.Tests.ModuleControllerTests.GetModulePropertyName_Action_Calls_GetModuleKindPropertyNames() in ModuleControllerTests.cs: line 213

I'm using RhinoMock as mocking tool. Can someone help with what mistake i'm making?

回答1:

After stubbing the method use Return to indicate what should it return, for example:

_mockRepository
  .Stub(x => x.GetModuleKindPropertyNames(Arg<string>.Is.Anything))
  .Return(Enumerable.Empty<string>().AsQueryable());

Also, change this line:

_controller.GetModulePropertyName(Arg<string>.Is.Anything);

to this:

_controller.GetModulePropertyName(string.Empty);

As the exception explains - Arg is only to be used in mock definitions.

回答2:

You don't have a return on your stub.

_mockRepository.Stub(x => x.GetModuleKindPropertyNames(Arg<string>.Is.Anything));

without that return, this line will be running a lambda against a null reference

 IList<Property> model = temp.Select(item => new Property {Name = item}).ToList();

so:

    _mockRepository.Stub(x => x.GetModuleKindPropertyNames(Arg<string>.Is.Anything)).Return(new Module[]{}); // set some return data here

来源：https://stackoverflow.com/questions/18273667/unit-testing-error-object-reference-not-set-to-an-instance-of-an-object