Why does casting the result from a controller in a unit test return null?

Question

I am in the process of equipping my sources with unit tests, but I made a mistake here. What can I do at this point if the value is really "null"? Is that even possible?

I have tried to figure out how others solve it but with the following thread I don't quite get along. C# in my unit test i want to see if it is null or an empty string

--> If I use IsNullOrEmpty() like in the example above, how does that look like in my code snippet?

This is the affected area in my unit test:

Assert.That(deviceInfo.SerialNumber == null);

The following error message appears:

Message: System.ArgumentNullException : Value cannot be null. Parameter name: source

Edit - This is what my environment looks like:

Here is the class with the DeviceDetails object which has all information in the body.

public virtual IActionResult DevicesIdGet([FromRoute][Required]string id)
{
    var device = _deviceManager.GetDevice(id);

    if (device == null)
        return NotFound();

    var deviceDetails = new DeviceDetails
    {
        IsOnline = device.IsOnline(),
        SerialNumber = null
    };

    return Ok(deviceDetails);
}

Unit-Test

 private Device _testDevice;

 [SetUp]
 public void SetUp()
    {
        _testDevice = new Device
        {
            Id = 1,
            DeviceType = 1,
            DisplayName = "TestDevice",
            IpAddress = IPAddress.Parse("127.0.0.1"),
            IpAddressString = "127.0.0.1"
        };
    }

[Test]
public void If_DeviceIsAvailable_Then_DeviceIdIsSelected()
{
    // ARRANGE
    var deviceManagerMock = new Mock<IDeviceManager>();
    deviceManagerMock.Setup(manager => manager.GetDevices(false))
        .Returns(new List<Device>
                 {
                     _testDevice
                 })
        .Verifiable();

    var subject = new DevicesApiController(deviceManagerMock.Object);

    // ACT
    var result = subject.DevicesIdGet("1");

    // ASSERT
    var deviceInfos = result as IEnumerable<DeviceDetails>;
    var deviceInfo = deviceInfos.Single();
    Assert.That(deviceInfo.IsOnline == true);
    Assert.That(deviceInfo.SerialNumber == null);
}

Answer 1

Your controller calls this method of IDeviceManager, which is not mocked:

var device = _deviceManager.GetDevice(id);

Not mocking that method is one cause for the exception. But if you fix it and mock that method (using Setup to specify a result on the mock) you'll still get the same exception. More on that later. First, here's the path your code is going down:

Because it's not mocked, the return value from calling GetDevice on the mock will be null, so this happens:

if (device == null)
    return NotFound();

The result of the method is a NotFound result.

Then, this is happening in the unit test:

var deviceInfos = result as IEnumerable<DeviceDetails>;
var deviceInfo = deviceInfos.Single();

result is a NotFoundResult. result as IEnumerable<DeviceDetails> returns null.

So you're effectively doing this:

IEnumerable<DeviceDetails> deviceInfos = null;
var deviceInfo = deviceInfos.Single();

The source parameter passed to the Single method is null, hence the exception.

---

If you mock GetDevice, you'll still get the same error for almost exactly the same reason. Now your code will return an OkObjectResult instead of a NotFoundResult. You'll try to cast that as IEnumerable<DeviceDetails>, it will still be null, and you'll get the same exception.

What you need is to get the value from the OkObjectResult, like this:

var actionResult = subject.DevicesIdGet("1") as OkObjectResult;
var deviceInfos = actionResult.Value as IEnumerable<DeviceDetails> ;
var deviceInfo = deviceInfos.Single();

Answer 2

On the basis of code posted, it appears deviceInfo can be null. You can you the following:

 if (deviceInfo == null)
      throw new ArgumentNullException(“source”);

 // rest of the code