ISerialized .Net, C#, Scrum and agile software development

How to unit test private methods in C# using NUnit

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:

private static object RunStatic(Type classType, string method)
{
    var methodInfo = classType.GetMethod(method,
    BindingFlags.NonPublic | BindingFlags.Static);
    return methodInfo.Invoke(null, null);
}

Given that I want to unit test the following class:

class ClassToTest
{
    private static bool MyTestMethod()
    {
        return true;
    }
}

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:

[TestFixture]
public class TestClass
{
    [Test]
    public void TestConcatTheseStrings()
    {
        var result = (bool) RunStatic(typeof(ClassToTest), "MyTestMethod");
        Assert.True(result);
    }

    private static object RunStatic(Type classType, string method)
    {
        var methodInfo = classType.GetMethod(method,
        BindingFlags.NonPublic | BindingFlags.Static);
        return methodInfo.Invoke(null, null);
    }
}

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

About Paul Eie

My name is Pål Eie (English alias: Paul Eie) I'm an IT consultant working in Stavanger, Norway. My experience spans from AIX to Windows and from embedded C to Uniface, Java, C++ and C#. My blog will involve everything that is related to technology. As long as it might be of interest to either newbies or seniors, I will blog about it! If you find some of my posts about basic old technology, it just means you know more than many of my other readers!