How to Test Private and Protected methods in .NET

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Introduction

Test Driven Development is the practice of (1) writing tests, (2) writing code that passes those tests, and (3) then refactoring. This concept is becoming very popular in the .NET community due to the quality assurance that it adds. While it is easy to test public methods, the natural question emerges "How do I test protected and private methods?"

This article will:

• Summarize some key points of the "Should-you-even-test private methods debate"?
• Make the case that it is still useful to at least know how to test private and protected methods - regardless of which side of the debate you're on.
• Provide the theory and downloadable code sample to demonstrate these testing techniques.

Background Theory

Should You Test Private Methods?

A Google search will show you that there's a lot of debate about using private methods, let alone testing them. The table below summarizes some of the common views of the pro and con for both issues.

	Pro	Con
Use private methods	Encapsulation - Private methods provide encapsulation, which makes the code easier for the end client to use Refactoring - It is easier to refactor private methods because they are never directly called by external clients, therefore changing the signature won't break any method calls. Validation - Unlike public methods that must validate all input because they are called by external clients, private methods are called safely within the class and don't require the same rigor of validation - the inputs should have been already validated in the public methods. Test Scope - Exposing every method as public greatly increases the test scope. Private methods are used only how the developer intended them, whereas public methods need to be tested for every possible case which required a broader test scope.	Didn't Refactor - If a class is complicated enough to merit private members, then it should be refactored. Hides Functionality - Private methods (if designed correctly) provide useful features that clients may want access to, so any private method worth testing should really be public.
Test private methods	Test Control - Private methods can contain complex logic and it increases test control to be able to directly access the method and test it, instead of needing to indirectly accessing it through a public method. Principle - Unit Testing is about testing the smallest functional piece of code; private methods are functional pieces of code, therefore on principle private methods should be testable.	Already Covered - Only the public interface should be tested. Private methods should already have thorough test coverage from being called by the public methods that are tested. Brittle Code - If you refactor the code, and juggle around the private methods, and if you had tests linked to those private methods, then you need to juggle around your tests too.

There are bright and experienced people on both sides of the issue. So while I have no intention or expectation of ending the "should I test private method" debate, there is still value for both sides to know how to test them. Even if you think that private methods should not be tested:

• Your opinion will be more influential if you can at least show that you are able to test them, but choose not to (i.e. you're not saying "Don't test private methods" simply because you don't know how).
• Having the option of testing non-public methods lets you see what really works best for your team over time.
• As long as there are still some valid situations, it's worth having a convenient way to test them.

Good Criteria and Inadequate Techniques

Andrew Hunt and David Thomas explain in their book, Pragmatic Unit Testing in C# with NUnit, that good unit tests are ATRIP:

• Automatic
• Thorough
• Repeatable
• Independent
• Professional

There are three additional criteria that any testing overhead for private/protected methods should meet:

• Transparency - Don't alter the System Under Test (SUT), such as adding wrapper methods in the production code.
• Scope - Be able to run in both Debug and Release mode.
• Simplicity - Have minimal overhead, and hence be easy to change and simple enough to introduce minimal risk.

Keeping these criteria in mind, there are several strategies that fall short:

Strategy	Problem
Don't have any private methods.	This avoids the issue.
Use the directives #if DEBUG ... #endif to wrap a public method which in turns wraps the private method. The unit tests can now indirectly access that private method through the public wrapper. (This is a method that I myself have used many times, and found it to be tedious and non-object oriented).	This only works in Debug mode. This is procedural and not object-oriented. We would need to wrap the individual methods in both the production code and unit tests. This alters the SUT by adding the public method wrappers.
Use the [Conditional("DEBUG")] attribute on public methods that wrap the private methods.	This only works in Debug mode.
Create internal methods to access the private method; then have a public test class elsewhere in the assembly that wraps those internal methods with public ones.	This alters the release code by adding the internal hooks, ultimately making the private methods accessible in production. This requires a lot of extra coding, and is hence brittle.

Testing Protected Methods

A protected method is visible only to derived classes, therefore it is not immediately available to a test suite. For example, suppose we wanted to test the method from ClassLibrary1.MyObject:

protected string MyProtectedMethod(string strInput, int i32Value) 
{
    return this.Name + ": " + strInput + ", " + 
     i32Value.ToString();
}

The book Pragmatic Unit Testing in C# with NUnit explains one solution: make a derived class MyObjectTester that inherits class MyObject, and then create a public method TestMyProtectedMethod that wraps the protected one. For example:

public new string TestMyProtectedMethod(string strInput, int i32Value) 
{
    return base.MyProtectedMethod(strInput, 
     i32Value);
}

This approach is simple, yet meets all the criteria:

Testing Private Methods

Testing private methods is a little more involved; but we can still do it using System.Reflection. You can use Reflection to dynamically access methods of a type, including both instance and static private methods. Note that accessing private methods does require the ReflectionPermission, but that is not a problem for Unit Tests running on a development machine or build server.

Suppose we wanted to test the private method MyPrivateMethod from ClassLibrary1.MyObject:

private string MyPrivateMethod(string strInput, DateTime dt, double 
 dbl) 
{
    return this.Name + ": " + strInput + ", " + 
     dt.ToString() + ", " + dbl.ToString();
}

One solution is to create a UnitTestUtilities project with a helper class to call the test method via reflection. For example, the download solution has the following methods in UnitTestUtilities.Helper:

public static object RunStaticMethod(System.Type t, string strMethod, 
 object [] aobjParams) 
{
    BindingFlags eFlags = 
     BindingFlags.Static | BindingFlags.Public | 
     BindingFlags.NonPublic;
    return RunMethod(t, strMethod, 
     null, aobjParams, eFlags);
} //end of method

public static object RunInstanceMethod(System.Type t, string strMethod, 
 object objInstance, object [] aobjParams) 
{
    BindingFlags eFlags = BindingFlags.Instance | BindingFlags.Public | 
     BindingFlags.NonPublic;
    return RunMethod(t, strMethod, 
     objInstance, aobjParams, eFlags);
} //end of method

private static object RunMethod(System.Type t, string 
 strMethod, object objInstance, object [] aobjParams, BindingFlags eFlags) 
{
    MethodInfo m;
    try 
    {
        m = t.GetMethod(strMethod, eFlags);
        if (m == null)
        {
             throw new ArgumentException("There is no method '" + 
              strMethod + "' for type '" + t.ToString() + "'.");
        }
                                
        object objRet = m.Invoke(objInstance, aobjParams);
        return objRet;
    }
    catch
    {
        throw;
    }
} //end of method

Private method RunMethod takes in the necessary parameters that Reflection needs to invoke a method, and then returns the value. It has two public methods that wrap this: RunStaticMethod and RunInstanceMethod for static and instance methods respectively.

Walking through RunMethod, it first gets the MethodInfo from a type. Because we expect this to only be called for existing methods, a null method triggers an Exception. Once we have the MethodInfo, we can invoke the method given the instantiated object (null for static methods) and the parameter array.

We could use this Utility in an NUnit test like so:

[Test] public void TestPrivateInstanceMethod()
{
    string strExpected = "MyName: Hello, 5/24/2004 
     12:00:00 AM, 2.1";
     
    ClassLibrary1.MyObject objInstance 
     = new MyObject("MyName");
    
    object obj = 
     UnitTestUtilities.Helper.RunInstanceMethod(
     typeof(ClassLibrary1.MyObject), "MyPrivateMethod",
     objInstance, new object[3] {"Hello", 
     new DateTime(2004,05,24), 2.1});
    
    string strActual = Convert.ToString(obj);
    
    Assert.AreEqual(strExpected,strActual);
}

Conclusion

While there is a debate on whether or not to test private methods, at least we have the ability to do so. We can test protected methods using inheritance to create a derived TesterClass that wraps the base protected methods with public ones. We can test private methods using Reflection, which can be abstracted to a UnitTestUtility helper class. Both of these techniques can help to improve test coverage.