Testing the Untestable with Visual Studio 2012 Fakes

Testing the Untestable with Visual Studio 11 FakesGaines Kergosienhttp://dotnetdude.net

Unit Testing• A unit test is a test of the correctness of an individual code module in isolation, such as a method.• The key to unit testing is isolation.

Code Isolation• Mock Object – Behavior verification• Method Stub – State verification• Method Shim – Method detouring

DelegatesUsing a delegate allows the programmer toencapsulate a reference to a method inside anobject. It can then be passed to code which cancall the referenced method, without having toknow at compile time which method will beinvoked.

Delegate Syntax

StubsStubs are concrete implementations ofinterfaces and abstract classes that can bepassed in to your system under test. Thedeveloper provides method implementations via.NET delegates or lambdas

InterfacesAn interface looks like a class, but has noimplementation. The only thing it contains aredeclarations of events, indexers, methodsand/or properties. The reason interfaces onlyprovide declarations is because they areinherited by classes and structs, which mustprovide an implementation for each interfacemember declared.

ShimsShims are run-time method interceptors. WithShims you can provide your ownimplementation for almost any method availableto your code in .NET, and that includes types andmethods from the .NET base class libraries.The shim library will create a Shim class forevery type in the Assembly, prepending theword Shim to the original name.

Non-Deterministic Results• Code is non-deterministic if it can exhibit different behaviors performing the same task• Tests need to be consistent• Reduce reliance on an outside environment

Stubs vs. ShimsFeature Stubs ShimsDetour mechanism Virtual Method Runtime InstrumentStatic methods, sealed types No YesInternal types Yes YesPrivate methods No YesStatic Constructors/Finalizers No YesPerformance Fast SlowerDeployment Xcopy InstallerAbstract methods Yes No

Steps to use Fakes1. Right click on reference to “Add Fakes Assembly”2. Use Fakes namespace.3. Control scope using ShimsContext in Microsoft.QualityTools.Testing.Fakes

ResourcesVisual Studio 2012http://aka.ms/vs11MSDN Documentationhttp://aka.ms/vs11-fakesDotNetDude Bloghttp://blog.dotnetdude.netSpeakerRatehttp://speakerrate.com/gainesTwitter@gainesk

Testing the Untestable with Visual Studio 2012 Fakes

by Gaines Kergosien

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