Asked 1 month ago by MartianSatellite846
Why Doesn’t the UNIT_TEST Conditional Compilation Symbol in the Test Project Affect the App’s Build in xUnit?
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I am using VS2022 Community Edition with xUnit to TDD a .NET 8 / .NET 4.8 WPF application. My goal is to disable a specific line of code in the app during test runs.
I added the following to the Tests project’s .csproj file:
XML<PropertyGroup Condition="'$(Configuration)'=='Debug'"> <DefineConstants>UNIT_TEST</DefineConstants> </PropertyGroup>
Then I wrapped the code to be skipped as follows:
CSHARP#if !UNIT_TEST Logger.Log($"EditorViewModel() instance: {GetHashCode()} : Text' {Text}'", "EditorViewModel"); #endif
However, the code still executes during testing. I expected to see the UNIT_TEST symbol in the app’s compilation output, but it isn’t present. Interestingly, if I add the same PropertyGroup to the app’s own .csproj file, it works as expected.
I suspect this is due to how conditional compilation symbols are applied only at the project level. Is this a bug with VS2022, or is there another explanation?
UPDATE:
Thanks to everyone for their input. I've learned a valuable lesson about TDD—I'm a total noob. I’m now looking into using MOQ as well.
I also considered a theoretical approach: since xUnit (or any test harness) compiles the app separately, it could potentially create a temporary copy of the app's .csproj file with an inserted UNIT_TEST define constant. As I’m already working on dynamic assembly generation and project file manipulation, this might be a direction to explore in building a custom testing harness.
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2 Answers
Answer by 
TychoAI Agent • 1 month ago
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When you add the define constant to your tests project’s .csproj file, it only affects the test project’s compilation—not the app under test. That’s why you don’t see any trace of UNIT_TEST in the app’s compiled output and the conditional code in the app isn’t disabled.
In other words, the app’s source code is compiled separately (using its own project file) so any conditional compilation symbols you define only in your tests project won’t change the app’s build. When you moved the PropertyGroup into the app’s .csproj file, the symbol was then defined during the app build, and the conditional code worked as expected.
The takeaway is that if you’d like to selectively disable code in your app during unit testing, you either need to define the constant in the app’s project file (or override it in the build process for test runs) or consider other techniques like dependency injection or mocking frameworks (e.g. MOQ) to bypass that behavior during tests.
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Answer by PlanetarySatellite872 • 1 month ago
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I was able to reproduce the issue you describe, and it does work as expected if I add the PropertyGroup to the application .csproj file.
This is because DefineConstants only applies to the current project and does not affect other projects. That is, you define UNIT_TEST in the .csproj file of the Tests project, but this is not automatically passed to the WPF App project. Preprocessing directives (such as #if !UNIT_TEST) are effective at compile time, and DefineConstants only applies to the project in which it is defined (i.e., your Tests project). The WPF App project is unaware of the existence of the UNIT_TEST constant, so the code is not excluded.
You can also refer to the description in this document:
If you're setting a property somewhere and not getting the expected
result, consider where and how the property is changed or used in all
the files imported by your project, including imports that are added
implicitly when you're using the attribute.
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