Creating shims

[testu...@gmail.com]

Hello All,

Can NSubstitute create what are called shims in Microsoft terminology?

For example, consider:

// Code under test

class MyClass

{

  public string str1;

  

  public string MyMethod()

  {

    AnotherClass thatThing = new AnotherClass();

    string str2 =  thatThing.DoSomething(str1);

    // more code, which is what I am really trying to test

    return result;

  }

}

// Unit test code

MyClass myThing = new MyClass();

myThing.str1 = "TestInput"

// When thatThing.DoSomething is called with input "SampleInput"

// it should just return "SampleOutput" instead of calling the

// real AnotherClass:DoSomething(string)

//

// Apparently Microsoft Fakes can do this by (I think)

// ShimAnotherClass.AllInstances.DoSomething = ("SampleInput) => "SampleOutput";

string actualResult = myThing.MyMethod();

Assert.AreEqual("ExpectedResult", actualResult);

[David Tchepak]

No NSubstitute doesn't support ShimX.AllInstances.

The usual approach to mocking this with NSubstitute would be to inject AnotherClass (or a factory if it needs to be created per call), or pass it to MyMethod. In addition, NSubstitute requires the mocked members to be virtual. 

Regards,

David

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[testu...@gmail.com]

Thanks, David. I have two questions:

1) You said that the mocked members had to be virtual. Is that true even if you are substituting an interface, as recommended in the docs? 

2) I have heard of dependency injection (which I assume is what you are talking about). I don't know anything about at this point, and I don't expect you to teach me here. However, can you *briefly* show how dependency injection will help me to mock AnotherClass? I *could* do something like (as you suggest):

// Unit test code

MyClass myThing = new MyClass();

myThing.str1 = "TestInput"

var thatThing = Substitute.For<AnotherClass>();

thatThing.DoSomething("SampleInput").Returns("SampleOutput");

string actualResult = myThing.MyMethod(thatThing);

Assert.AreEqual("ExpectedResult", actualResult);

but I would rather not if I can help it. thatThing is an implementation detail that does not need to be visible outside of MyClass::MyMethod. It would be in the argument list just to make it accessible to NSubstitute.

[David Tchepak]

1) Short version: interfaces are fine too. Better in fact!

Long version: 

Sorry I should have been clearer. NSub (and other DynamicProxy-based libraries) work by sub-classing the type you're substituting for, and overriding each member to change how it works. All members of an interface can be overridden, so that works best. Classes and abstract classes will also work provided the members are abstract or virtual (i.e. they can be overridden in the sub-class). You can also substitute for classes and abstract classes with a mix of virtual and non-virtual members, but NSubstitute will not know when the non-virtual members are called and so won't be able to change their behaviour (in fact, trying to set return values on non-virtuals can cause NSub to get confused and make your test fail).

2) Here are some examples of injecting the dependency into the code being tested: https://gist.github.com/dtchepak/8948799

Let me know if you would like explanations of any of them.

Hope this helps.

Regards,

David

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[testu...@gmail.com]

2) Here are some examples of injecting the dependency into the code being tested: https://gist.github.com/dtchepak/8948799

Hello David,

Your examples are clear. Basically, they are variations of  the same theme - I must move the creation of thatThing from inside MyClass::MyMethod to the code that calls MyClass::MyMethod. I think that I will pass thatThing via the MyClass constructor. That way I don't clutter MyMethod's argument list. A shim would have been better, though. Is it possible that shims might be implemented in NSubstitute one day?

[David Tchepak]

It's very unlikely - NSubstitute isn't really designed for that. MS Fakes or something like TypeMock or JustMock are better bets if you need very advanced interception capabilities like this.

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