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

C# unit testing private methods: avoiding runtime errors

In C#, you can use Visual Studio's automatically generated accessor classes to unit test private methods. However, runtime errors may occur when trying to manipulate the object's internal state through these tests.

Fundamental problem

The error encountered in the provided code is due to the accessor class target being treated as TypeA_Accessor at compile time, but treated as TypeA at runtime. This mismatch causes the Add() operation on target.myList to fail because the list types are different.

Resolving errors

In versions prior to .NET Core 2.0, you can use the PrivateObject class to access private methods. This requires the following steps:

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

Alternative methods

Instead of testing private methods directly, consider the following alternative:

• Testing a public method that calls a private method: By testing the behavior of a public method, you can indirectly verify the functionality of the internal method.
• Refactor to remove private methods: If feasible, extract private methods into a new public or protected class so they can be tested directly.
• Use Dependency Injection: Create abstractions for internal components and inject them into the class under test to facilitate test replacement.

Please note that PrivateObject and PrivateType support have been removed in .NET Core 2.0 and later, requiring alternative methods to access and test private methods in newer versions of the framework.

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