Unit testing private methods in C# using NUnit and Reflection

Ever had the need to unit test a private method? Ever change a private method to public just to able to write a test for it? You are definitely not alone! A couple of weeks back I had a very interesting discussions with a very good colleague of mine: "What to do if I want to unit test a private method in C#". In this post I will show how you can use NUnit in combination with Reflection to unit test private methods!

Whether unit testing private method is wrong or not, is far bigger discussion which I will not get into details on in this post. Some of the criticism I have met when I started looking into this, was that if I saw the necessity to unit test a private method, it only meant that my design was wrong! I see the point, but I still see many situations where I either of historical or architectural reasons still have the need to write a unit test for a private method.

I could of course change the method to public, but when working on a larger projects with many junior developers, it's easier to force developers to use a class correct if I only expose the necessary methods for the class to work correctly!

The solution I found to this was to write a method that uses Reflection to extract a MethodInfo object, and then invoke on this to execute the method. So to execute a private static method I can write the following method:

   1:  private static object RunStatic(Type classType, string method)

   2:  {

   3:      var methodInfo = classType.GetMethod(method, 

   4:                  BindingFlags.NonPublic | BindingFlags.Static);

   5:      return methodInfo.Invoke(null, null);

   6:  }

Given that I want to unit test the following class:

   1:  class ClassToTest

   2:  {

   3:      private static bool MyTestMethod()

   4:      {

   5:          return true;

   6:      }

   7:  }

Using Reflection I can write a unit test on MyTestMethod even though it is a private method, by using my RunStatic method. Hence me the following unit testing class:

   1:  [TestFixture]

   2:  public class TestClass

   3:  {

   4:      [Test]

   5:      public void TestConcatTheseStrings()

   6:      {            

   7:          var result = (bool) RunStatic(typeof(ClassToTest), "MyTestMethod");

   8:          Assert.True(result);

   9:      }

  10:   

  11:      private static object RunStatic(Type classType, string method)

  12:      {

  13:          var methodInfo = classType.GetMethod(method,

  14:              BindingFlags.NonPublic | BindingFlags.Static);

  15:          return methodInfo.Invoke(null, null);

  16:      }

  17:  }

Once again: This is a work around! Don't kill the messenger ;)