Overview [ Documentation]

Availability: Visual Studio Enterprise Edition, Visual Studio 2019 Update 6+

Code isolation is a testing strategy that isolates code under test from the rest of the application. This allows for testing the specific functionality of the code in isolation. If a test fails, the cause is in the isolated code, not somewhere else. It also enables testing code even if other parts of the application are not yet functional.

How it Works

Microsoft Fakes achieves code isolation using stubs and shims:

• Stubs — used to replace a class with a small substitute that implements the same interface.
  • Requires application design such that each component depends only on interfaces, not other components.
  • Typically, used for calls within a solution.
  • Some external assemblies do not provide interface definitions, so shims needs to be used instead.
  • Pros:
    • Stubs can be generated from interface definitions in Visual Studio.
  • Cons:
    • Stubs cannot be used for static methods or sealed types (since they cannot have an interface).
    • Can only replace visible (non-private) methods.
• Shims — used to modify the compiled code at runtime. Instead of making a specified call, the application runs the provided shim code. Shims replace calls to assemblies that are not in your control.
  • Typically, used for calls to other referenced assemblies.
  • Pros:
    • Can replace calls to private methods if all the types on the method signature are visible.
  • Cons:
    • Shims run slower than stubs because they rewrite code at runtime.

Creating a Microsoft Fakes Assembly

1. Solution Explorer > TEST_PROJECT_NAME > [Assemblies, Projects or Packages]
2. Right-click the assembly that contains class definitions to be stubbed > Add Fakes Assembly