Should I Unit Test Private Methods in C#, and How Can I Avoid Runtime Errors?

Private methods in C# unit testing: runtime errors and solutions

Visual Studio's ability to unit test private methods via auto-generated accessor classes may seem convenient, however, in some cases such tests can encounter runtime errors, as shown in the provided sample code. The problem stems from a difference between the type inferred by the compiler (TypeA_Accessor) and the actual runtime type (TypeA), resulting in a conversion error when adding elements to the list.

To resolve this issue, consider the following:

• Using PrivateObject (available before .NET Core 2.0): The PrivateObject class allows access to an object's private members, including methods. It can be used like the example provided in the question answer. However, please note that support for PrivateObject has been removed in .NET Core 2.0.

Alternatively, please consider the following general advice:

• Avoid testing private methods: Testing private methods can lead to brittle tests that may break when internal implementation details change. Instead, focus on testing the public interface of your class.
• Avoid manipulating object state in unit tests: Unit tests should generally verify specific behavior without changing the state of the object being tested. This ensures that subsequent tests are not affected by the results of previous tests.

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