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?

http://stackoverflow.com/a/17574183/37055 is an option if you're willing to include an internal parameterless constructor. Which honestly I find really reasonable to almost do it as a rule. — Chris Marisic, Apr 13 '16 at 19:13
Nsubstitute combined with AutoFixture should be able to accomplish this. http://blog.ploeh.dk/2013/01/09/NSubstituteAuto-mockingwithAutoFixture/ — Jack Ukleja, Nov 16 '16 at 01:26

score 9 · Accepted Answer · edited May 23 '17 at 12:33

9

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.

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edited May 23 '17 at 12:33

Community

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answered Jun 19 '13 at 05:59

David Tchepak

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Yep, seems that not all the members of the concrete class are virtual. Seems like reflecting over the substitute may be the best solution available. – karmasponge Jun 19 '13 at 08:11
I wrote a reflection sample a while back if that helps as a starting point: https://gist.github.com/dtchepak/3348663 (I updated my answer to include this link) – David Tchepak Jun 19 '13 at 10:56
Thx, in my case I'll want to expand it to handle indirect properties, properties that return interfaces that return the concrete class. – karmasponge Jun 26 '13 at 01:14

ClickRick · Answer 2 · 2014-05-15T22:13:17.473

It doesn't count as auto-mocking but you did also ask "Or is there a way to override the behaviour?" and "Perhaps there is a simpler solution that I have over-looked?"

This answer relies on the statements in your question that:

SomeObj is a class outside of your control, from which I assume it is either separately tested or else not testable
SomeObj has a default constructor

Sure, it requires you to "manually initialise the mock before the test" but since you've not told us what this object is it's not possible to know how much work it would take to implement fully.

public class SomeObj
{
    // Non-virtual to prevent auto-mocking
    public void Dummy() { }
}
public interface ISomething
{
    SomeObj First { get; }
    SomeObj Second { get; }
}
[TestMethod]
public void Test_17182355ms()
{
    ISomething mockedSomething = Substitute.For<ISomething>();

    SomeObj firstObj = mockedSomething.First;
    Assert.IsNull(firstObj);
    mockedSomething.First.Returns(new SomeObj());
    mockedSomething.Second.Returns(new SomeObj());
    firstObj = mockedSomething.First;
    Assert.IsNotNull(firstObj);
}

Another approach, though not without its own drawbacks, would be to extract your own interface for SomeObj, something like this:

public interface ISomeObj
{
    void Dummy();
}
public class MySomeObj : SomeObj, ISomeObj
{
}

and then mock ISomeObj in your test.

Can I get NSubstitute to auto mock my concrete classes?

2 Answers2

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