Question

I have a class that has a property that I need to stub. I can’t pass it as part of the constructor because the object constructing it does not know the parameters of the constructor.

When running unit tests, I want to be able to have the property be created as a stub.

This is what I have tried, but it does not work:

private DeviceMediator deviceMediator;
private IDeviceControlForm deviceControlForm;
private IDataAccess data;
private ICallMonitor callMonitor;

// Use TestInitialize to run code before running each test 
[TestInitialize()]
public void MyTestInitialize()
{
    // This line works fine
    deviceControlForm = MockRepository.GenerateStub<IDeviceControlForm>();          
    // This line works fine
    data = MockRepository.GenerateStub<IDataAccess>();
    // This has to be an ICallMonitor.  If I try to make it a 
    // CallMonitor then it fails.
    callMonitor = (CallMonitor)
      MockRepository.GenerateStub<ICallMonitor>();
    // This line does not compile.  Because it wants to 
    // return a CallMonitor not an ICallMonitor.
    Expect.Call(new CallMonitor(null)).Return(callMonitor);

    // This is the class that has the CallMonitor (called callMonitor).
    deviceMediator = new DeviceMediator(deviceControlForm, data);
}

Is there anyway to catch the constructor call to CallMonitor and make it actually be a stub?

In case it is relevant, here is the related code in DeviceMediator:

 private IDeviceControlForm form;
 private readonly IDataAccess data;
 public ICallMonitor CallMonitor { get; set; }

 public DeviceMediator(IDeviceControlForm form, IDataAccess data)
 {
     this.form = form;
     this.data = data;
     CallMonitor = new CallMonitor(OnIncomingCall);
 }

Thanks in advance for any help.

Answer 1

Since the CallMonitor property is writable, you can just overwrite the original value with a mock instance (your DeviceMediator actually implements the Property Injection design pattern).

So you can write a test like this:

[TestMethod]
public void MyTest()
{
    var deviceControlForm = MockRepository.GenerateStub<IDeviceControlForm>();
    var data = MockRepository.GenerateStub<IDataAccess>();
    var mockCallMonitor = MockRepository.GenerateStub<ICallMonitor>();

    var deviceMediator = new DeviceMediator(deviceControlForm, data);
    deviceMediator.CallMonitor = mockCallMonitor;

    // The rest of the test...
}