Starting with the release of Visual Studio 2012 Update 2 the Fakes framework is now available to use in both the Premium and Ultimate versions! HURRAY! Why am I so excited? Because this will allow me to unit test my code.

Now I know everyone doesn’t get as excited about unit testing code as I do but that’s ok. By the end of this 2 part series hopefully you will at least be a little excited, and realize that it isn;t as big of a chore as you may think.

The Fakes framework is based off of the moles and pex frameworks that we have been using for a while now. So what’s new in fakes? In terms of general usage, not a whole lot a lot of the changes are under the hood.

The biggest differences are:

  1. You don’t need to use the HostType Attribute
  2. Detours are referred to as Shims instead of Moles.
  3. Moles files are now fakes files

So a lot of things seem pretty cosmetic.

In this first post I will be talking about stubs, but before we do that let’s quickly discuss the difference between stubs and shims. You would use a stub when you are using a dependency injection pattern combined with sort of IOC container such as Unity.

You use a shim when you want to override the behavior of code that you really don’t have control over; something that you can’t pass in. So if you wanted to change the behavior of an inbuilt .Net method you could use shims to do that.

Using Stubs

In my example I have the following business logic code that I want to test out.

  public class PersonManager
        private IPersonData _personData;
        public PersonManager(IPersonData personData)
            _personData = personData;
        public int Save(Person p)
            p.Name = "Changed Name";
            return _personData.SavePerson(p);

I am injecting my data access code in the constructor above, so this is a great example of when I want to use a stub. The code that I want to test is the Save method and as you can see there isn’t a whole lot there. But that is fine for our example.

In order to test this I first need to create my unit test project, just as you always have no changes there. The next step is to add all of your assemblies that you will need. In this case I need my data contracts, data access and my business logic layers in order to get this example working but the only thing that I need to fake would be my data access code since I want to isolate my business logic. In order to do that, assuming you have fakes installed, right click on your reference and click on add fakes assembly. Easy as that. I am now ready to write my unit test.

using System;
using Data;
using Data.Fakes;
using DataContracts;
using FakesTemp; //Name of my Buiness logic project
using Microsoft.VisualStudio.TestTools.UnitTesting;
namespace BusinessLogic.Tests {
    public class PersonTests {
        public void PersonTests_HappyPath()
            IPersonData personData = new StubIPersonData {
                SavePersonPerson = person => { return person.Age; }

            PersonManager target = new PersonManager(personData);
            var p = new Person
                    Age = 28,
                    Name = "Bruce Campbell"
            int expected = 28;
            int actual = target.Save(p);
            Assert.AreEqual(expected, actual);

If you have been using Moles for a while, this shouldn’t look very different to you. The only major difference would be using the StubIPersonData instead of SIPersonData that you would see if you were using Moles.

Taking a closer look at StubIPersonData we can see that we are stubbing out the SavePerson method that my business logic above is calling. So why is it called SavePersonPerson? The second ‘Person’ is the type of argument I am passing in. For example if I was passing a string into the method the stub would look like SavePersonString.

The next part is where I am declaring the parameters I am passing in, in this case person. This will catch whatever I am passing in and I can then manipulate the object in my stub if I need to. In this example I am just going to return whatever the person’s age is just to prove that point, but you can return any integer that you wish, since that is the return type of the SavePerson method.

From there you just create the class you are testing, by convention named target, and pass in your stubbed out IPersonData interface. Then create a Person object to pass into my save method, and then actually call the save method and capture the result. I am expecting to get back the age of the person, so I want to assert that is actually what I am getting back.

Taking it further

So in the example above, I am only stubbing out one method in one class, but you can stub any number of classes and any number of methods inside of those classes. Just because the method exists in your interface however doesn’t mean that you have to stub it if the method you are testing doesn’t use it. There is no reason to add clutter in your code.


  • Fakes are now available to users in both the Ultimate and Premium versions of Visual Studio 2012 assuming you have update 2 installed.
  • Fakes assemblies allow you to either create stubs or shims
  • Stubs allow you to override implementations of classes you pass in using an IOC container or something similar.
  • Shims allow you to override built in classes (or classes you don’t have control over) at run time.
  • Unit testing is AWESOME!

Look for part two of this series where we are going to talk about using shims!

This is also posted on my company blog!

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