Proper unit testing involves identifying the smallest pieces of testable code and ensuring that they function properly. Testing these units effectively generally requires somehow isolating them from the rest of the code base. There are a number of frameworks that help with this isolation but the Fakes isolation framework goes beyond interfaces and virtual methods by allowing us to detour any framework method. In this session we’ll explore how stubs and shims can help us isolate testable units regardless of whether we control the dependencies.

1. Isolating Dependencies with Microsoft Fakes

2.  Introducing Fakes
 The Basics
 Using Stubs
 Using Shims
 Advanced Configuration
 Final Notes
 Resources

3.  What is it?
 Where did it come from?

4.  Inheritance based
 Used when testing a defined contract

5. Let me start by saying Shims are evil. But they are
evil by design. They let you do things you otherwise
couldn’t do, which is very powerful. They let you do
things you might not want to do, and might know
you shouldn’t do, but because of the real world of
software, you have to do.
Peter Provost
Program Manager Lead
Visual Studio ALM Tools

6.  Use a profiler to rewrite existing method
bodies
 Detour virtually any .NET method
 Performance issues due to the profiler

7.  No additional installation required
 Isolation type generation driven by .fakes file
 Microsoft.QualityTools.Testing.Fakes
assembly/namespace

8.  Delegates are used to provide the faked
functionality
 Custom delegate types including support for
out and ref parameters
 Static class:
 Microsoft.QualityTools.Testing.Fakes.FakesDelegates

9.  Action  Func
 OutAction  OutFunc
 OutOutAction  OutOutFunc
 RefAction  RefFunc
 RefRefAction  RefRefFunc

10.  Isolation types are placed in a .Fakes namespace
 Stub types are prefixed with “Stub”
 Shim types are prefixed with “Shim”
 Method parameter types are appended to the
method name
 “Get” or “Set” is appended to property names
 Setters also include parameter type names (like
methods)
 Generic type arguments are appended to the
method name for generic methods
 Constructors have parameter type names
appended to “Constructor”

11.  Assign a delegate to the field that
corresponds to the member being isolated

12.  Same process as methods
 Get and Set can be assigned individually
 AttachBackingFieldTo[PropertyName]
convenience method

13.  The framework generates a public method to
invoke events
 [EventName]Event

14.  Allow calling the base implementation when
a delegate is not set for a member

15.  Control what happens when a delegate has
not been provided for a member
 Set through the InstanceBehavior property

17.  Shim properties for static members are static
on the shim type
 Static AllInstances property for applying
delegates to all instances of the shim type
 Instance properties for applying delegates to
a single instance

18.  ShimsContext
 Sets up the shim under the IntelliTrace profiler
 ShimsContext.Create()

19.  Events don’t have a method to invoke
 Properties representing the Add and Remove
operations are provided instead
 Base type members aren’t directly accessible
 Static constructors are “shimmable”
 Bind method for each interface implemented
by the shimmed type
 Behaviors are controlled by ShimBehaviors

21.  Controlling Type Generation
 Strong Name Signing

22.  Generating the isolation types is expensive
and will extend build time
 Configure through the .fakes file
 Filtering

23. Attribute Type Stubs? Shims?
AbstractClasses Boolean 
FullName String  
Interfaces Boolean 
Namespace String  
Obsolete* Boolean  
TypeName String  
* Applies only to Remove elements

24.  ! forces an exact, case sensitive match
 * matches the beginning of the string

25.  Fakes assemblies will be signed with the
same key as the underlying assembly unless
otherwise specified
 Compilation element’s KeyFile attribute in
.fakes file

26.  Beware the Docs!
 Notable Changes from Moles

27.  Configured through .fakes file rather than .moles
 Namespace and assembly are .Fakes rather than
.moles
 Mole types are now shim types
 Stub prefix is “Stub” rather than “S”
 Shim prefix is “Shim” rather than “M”
 ShimsContext replaces HostTypeAttribute
 Automatic properties can be isolated!
 Moles allowed for erasing static constructors and
finalizers. Fakes allows shimming static constructors
but has no support for finalizers
 Moles could control the bitness of the profiler
 “classes” filter attribute was removed

28.  Isolating Unit Test Methods with Microsoft
Fakes
http://bit.ly/KYMkyX
 Moles Isolation Framework for .NET
http://bit.ly/LebqZ2
 Going Underground: Microsoft Moles
http://bit.ly/y66D5C