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    <p>If you expose the injected <code>clientID</code> (and <code>dependency</code> as well) as read-only properties, you can always query their values:</p> <pre><code>public class BuilderStrategy { private readonly string _clientID; private readonly IDependency _dependency; public void BuilderStrategy(string clientID, IDependency dependency) { _clientID = clientID; _dependency = dependency; } public string GetClientExtension() { return _clientID.Substring(_clientID.LastIndexOf("-") + 1); } public string ClientID { get { return _clientID; } } public IDependency Dependency { get { return _dependency; } } } </code></pre> <p>This <a href="http://blog.ploeh.dk/2013/04/04/structural-inspection" rel="noreferrer"><em>doesn't</em> break encapsulation, but is rather known as Structural Inspection</a>.</p> <p>With this change, you could now rewrite the test like this:</p> <pre><code>[Theory, MyConventions] public void GetClientExtensionReturnsCorrectValue(BuilderStrategy sut) { var expected = sut.ClientID.Substring(sut.ClientID.LastIndexOf("-") + 1); var actual = sut.GetClientExtension(); Assert.Equal(expected, actual); } </code></pre> <p>Some people don't like duplicating production code in the unit test, but I would rather argue that if you follow Test-Driven Development, it's the production code that duplicates the test code.</p> <p>In any case, this is a technique known as <a href="http://xunitpatterns.com/Derived%20Value.html" rel="noreferrer">Derived Value</a>. In my opinion, <a href="http://blog.ploeh.dk/2013/04/02/why-trust-tests" rel="noreferrer">as long as it retains a cyclomatic complexity of 1, we can still trust the test</a>. Additionally, as long as the duplicated code only appears in two places, the <a href="http://en.wikipedia.org/wiki/Rule_of_three_%28computer_programming%29" rel="noreferrer">rule of three</a> suggests that we should keep it like that.</p>
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    1. POCan a particular constructor argument of an injected SUT using Autodata xUnit Theories?
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    1. USMark Seemann
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    1. COThanks, Mark, also for the embedded references. In using Structural Inspection, should the readonly properties be part of the interface or only the implementations under test (assuming there exists an `IBuilderStrategy` which `BuilderStrategy` implements)? Also, I would think that it would be better to expose these properties *as the need arises* in unit testing, rather than expose all dependencies as a routine matter (although, I suppose that in practicing TDD, this would all happen naturally). Do you have an opinion on any of that?
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    2. COUsing Derived Value it seems possible to get [False Negative](http://xunitpatterns.com/false%20negative.html) results if your tests are written in C# and your SUT is written in VB.NET. In my code, R# suggested using `StringComparison.Orginal` with `LastIndexOf` in the C# test, but not in the VB.NET-written SUT. The R# suggestion isn't the problem, but it indicates that using Derived Value either tightly couples you to a particular .NET language, or expose you to [sneaky differences that exist between languages](http://stackoverflow.com/a/14931907/533958)
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    3. COThe property is part of the concrete class: "Ultimately, adding a property to a concrete class has no impact on the exposed interface."
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