How Can I Effectively Unit Test Private Methods in C#?

Unit Testing Private C# Methods: Challenges and Solutions

Directly unit testing private methods in C# is problematic due to their encapsulation. While Visual Studio generates accessor classes to aid in this, runtime errors, such as object conversion failures when interacting with private lists, can occur. This often stems from type mismatches between the accessor class and the target class.

The Issue:

A common scenario is a unit test failing during execution because of an object conversion error when trying to add an item to a private list. The accessor class's list type differs from the original class's list type.

Addressing the Problem:

Deprecated Approach: PrivateObject Class:

In versions of .NET prior to .NET Core 2.0, the PrivateObject class offered a way to call private methods:

<code class="language-csharp">Class target = new Class();
PrivateObject obj = new PrivateObject(target);
var retVal = obj.Invoke("PrivateMethod");
Assert.AreEqual(expectedVal, retVal);</code>

Important: PrivateObject and PrivateType are no longer supported in .NET Core 2.0 and later.

Better Alternatives:

The best practices generally discourage direct testing of private methods. Here's why and what to do instead:

• Indirect Testing via Public Methods: Focus on testing the public interface. If your public methods work correctly, the private methods supporting them are likely also correct.
• Avoid State Changes in Tests: Unit tests should ideally not alter the internal state of the object under test. Concentrate on verifying the output based on specific inputs.
• Employ Mock Objects: Mocking provides a controlled environment to test the interactions and outputs of private methods without direct access.
• Refrain from Using Reflection: While reflection allows access to private members, it's complex, error-prone, and generally not recommended for unit testing. It makes tests brittle and harder to maintain.

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