TestAccessor Pattern.md

The TestAccessor Pattern

The TestAccessor pattern allows production code to expose internal functionality for test purposes without making the internal functionality available to other production code. The pattern has two primary components:

1. A TestAccessor type, which contains the functionality available only for testing
2. A GetTestAccessor() method, which returns an instance of TestAccessor

The pattern relies on enforcement of a simple rule that no production code is allowed to call a GetTestAccessor() method. This is enforceable either through code reviews or through an analyzer. This pattern has many advantages over alternatives:

• The pattern does not require expanded accessibility (e.g. internal instead of private) for the purpose of testing
• The pattern is self-documenting: all properties and methods within a TestAccessor type are intended only for use in test code
• The pattern is consistent enough to enforce through static analysis (analyzers)
• The pattern is simple enough to enforce manually (code reviews)

The TestAccessor Type

The TestAccessor type is typically defined as a nested structure. In the following example, the SomeProductionType.TestAccessor.PrivateStateData property allows test code to read and write the value of the private field SomeProductionType._privateStateData without exposing the _privateStateData field to other production code.

internal class SomeProductionType
{
  private int _privateStateData;

  internal readonly struct TestAccessor
  {
    private readonly SomeProductionType _instance;

    internal TestAccessor(SomeProductionType instance)
    {
      _instance = instance;
    }

    internal ref int PrivateStateData => ref _instance._privateStateData;
  }
}

The GetTestAccessor() Method

The GetTestAccessor() method is always defined as follows:

internal TestAccessor GetTestAccessor()
  => new TestAccessor(this);