Stubbing a method which is called in class constructor in RhinoMocks

Question

My class has a method like this:

internal virtual Something CreateSomething(){...}

It was done this way so that in testing I can stub CreateSomething() to return a mock object:

var something = ...; //some mock object
var t = MockRepository.GenerateStub<MyObject>();
t.Stub(x => x.CreateSomething()).Return(something);

This worked fine but now CreateSomething() is called in the class constructor, before it was called later on, so by the time I stub the method my object is already created.

Is there a workaround that doesn't involve changing the design, to pass stub methods in at construction time? If not I can consider changing the design to use construction-injection (which I'm sure some of you are itching to suggest anyway!) but I'd rather see if Rhino supports this use-case first.

Answer 1

There is no way to stub the method before it is invoked from its class constructor. This is C# limitation. And this is reasonable.
In general it is a bad practice to call virtual method from the constructor, because likely the result will not be what is expected. See details here: Virtual member call in a constructor.

So, I highly recommend to either make this method non-virtual, or avoid its invocation from the constructor.
Anyway in such situation it won't work as virtual.

Regarding you question about testing the method, which is called from constructor.
I presume, this is a private method. Otherwise it could be tested as a regular public method.
As it is private, then there is no need to test this particular method in isolation. I recommend to follow general approach in testing constructors and testing private methods: Do not test private methods.
Test the public visible effect instead. That could be:

• Public property initialization
• Mocked dependencies invocation or properties access
• Exception thrown
• etc.

Please also find more details about this topic by the links below:
Is it important to unit test a constructor?
Unit testing private methods in C#