Can I get NSubstitute to auto mock my concrete classes?

Question

I have an interface which I am mocking with 'NSubstitute' which contains properties that return concreate classes, that is the return value is not an interface. e.g

public interface ISomething
{
    SomeObj First { get; }
    SomeObj Second { get; }
}

The 'SomeObj' concrete class has a default constructor but 'NSubstitute' always returns 'null' for these properties. The class itself is not under my control so I cannot simply make it derive from an interface.

Can 'NSubstitute' mock these type of properties? Or is there a way to override the behaviour? Otherwise I have to manually initialise the mock before the test and that can be a lot of code (even if its reused through a common method).

Perhaps there is a simpler solution that I have over-looked?

Answer 1

Classes will be auto-mocked if they have a default (parameterless) constructor and all its members are virtual (see the note in the intro of Auto and recursive mocks). The aim of this is to reduce the potential for unwanted (destructive?) side-effects if we are using a substitute and suddenly hit a non-virtual, unmocked code path that does bad stuff in an instance we thought was fake.

NSubstitute doesn't have a way override this behaviour. Instead, I'd recommend creating all your substitutes via your own factory method (e.g. a static Sub.For<T>(...) method in your test project) that uses NSubstitute to produce a substitute, then applies all the specific initialisation rules you need, like using reflection to stub out values for each class property.

Hope this helps.

Possibly related links:

• I advise trying to avoid mocking types we don't own.
• Stack Overflow: Is it recommended to mock concrete class?
• Hacky factory method sample that subs properties using reflection.