if I have a class with a helper (private member) class within it, like this
class Obj;
class Helper {
friend class Obj;
private:
int m_count;
Helper(){ m_count = 0;}; // Note this is a private constructor
void incrementCount(){
++m_count;
};
};
class Obj {
Helper *m_pHelper;
// note that this is a private getter
int getHelperCount() { return m_pHelper->m_count; };
// the public API starts here
public:
Obj() { m_pHelper = new Helper(); };
void incrementCount(){ m_pHelper->incrementCount(); };
};
So how may I TDD such a system?
auto obj = new Obj();
obj->incrementCount();
// what to assert???
That is my question and the following is just some background.
Response to some answers and comments.
If noone outside the class should be interested, then your tests should not be interested either. – Arne Mertz
If nobody is interested in the value outside the class, why are you – utnapistim
Even if no one outside needs the value, I may still want to know that if it's set correctly, as it is used by other self contained internal method of the class that use that value. Maybe the value is the speed where the controller will use it to update the model. Or maybe it's the position where the view will use it to draw something on the screen. And in fact all other components of Obj would be able to access that variable. It may be a bad design issue, and in this case I would like to know what better alternatives I can have. The design is listed in the background section at the bottom of this post.
define private public - Marson Mao
Love this ingenious abuse of keywords haha. But may not be concluded as the best solution just yet.
You need to "expose" the friendship relation in the header of your class. Thus you have to acknowledge there the existence of a class used to test yours.
If you use the pImpl idiom, you could make the members of the pImpl itself all public, the pImpl itself private and give your unit tests access to the pImpl - CashCow
Does this mean that I should friend the test in my original class? Or add extra "test" methods to it?
I just started TDD very recently. Is it common (or better is it good) to intrude the original class with test class dependency? I don't think I have the appropriate knowledge to judge. Any advice on this?
Miscellaneous: AFAIK TDD is not just writing test, but instead a development process. I have read that I should only write tests to the public interface. But the problem is, like the situation in question, most of the codes etc are contained within private class. How may I use TDD to create these codes?
Background
FYI if you would like to know why I am making a private class:
I am developing a game from cocos2dx. The game engine adopts a Node tree structure for the updates, rendering etc and every game object would inherit from a Node class provided in the engine. Now I want to implement the MVC pattern on a game object. So for each object I basically created a Object class with 3 helper classes corresponding to each of the MVC components named ObjectModel, ObjectView, ObjectController. Theoretically no one should access the MVC classes directly and would only be accessed somehow through the Object class so I make the 3 of them private. The reason of making the MVC components explicitly as classes is because the View and Controller are updating at different rates (more specifically the Controller performs frame dependent updates, while the View do a simple interpolation based on the model data). The Model class is created purely for religious reasons lol.
Thanks in advance.
How to test friend functions of private class?
Thou shalt not!
A class (or module or library or whatever) exposes a public interface for a reason. You have the public interface (which is geared for client use, so it has invariants, preconditions, postconditions, side-effects, whatever - which can and should be tested) and implementation details, that allow you to implement the public interface, easier.
The point of having a private implementation, is that you are allowed to change it as you please, without affecting other code (without affecting even tests). All tests should pass after you change your private implementation, and client (and test) code should (by design) not care at all that you changed the private implementation.
So how may I TDD such a system?
TDD your public interface only. Testing implementation details means you end up coding to an implementation, instead of an interface.
Regarding your comment:
The problem is I don't even have a getter in the public interface. So how can my test check that the value is 0 or 1? And the getter is intentionally made private as no one should be interested in the value outside the class
If nobody is interested in the value outside the class, why are you (i.e. why would you wish to test for it?)
The #define private public trick can have side effects with the way some compiler are mangling function symbols (Visual c++ compiler is including access specifier in its name mangling)
You can also change visibility with the using statement :
struct ObjTest : public Obj
{
using Obj::incrementCount;
}
But like other people said, try to not test private stuff if possible.
I have encounter such problem when I was writing unit test as well.
After some searching I decided the most effective way is to add this in your Test.cpp:
#define private public
NOTE: add this before your desired include file, maybe your Obj.h, for example.
I think this method looks crazy but it's actually reasonable, because this #define only affect your test file, so all other people using your Obj.h is totally fine.
Some reference:
Unit testing of private methods
I vote, as #Marson Mao says, for #define private public.
If you want to control what to make private or public a bit more, you can do this in yourtests.cpp
#define private public
#include "IWantAccessViolationForThis.h"
#undef private
#include "NormalFile.h"
This way you can have a bit more control and try to do this trick in as few places as possible.
Another nice property of this approach is that it is non-intrusive, meaning that you don't need to clutter your real implementation and header files with #ifdefs for testing and not testing modes.
Your friend has full access to the class that it is a friend of. This might be done for many reasons and one of those could well be for unit-testing purpose, i.e. you want to be able to write a unit test that can call private members of the class and check the internal variables show what you would expect them to show, but you do not want that to be part of the public API.
You need to "expose" the friendship relation in the header of your class. Thus you have to acknowledge there the existence of a class used to test yours. No worries, you develop in the real world and classes are tested.
In order to write a unit test you will want to implement that class to provide protected member functions (probably static ones) that call all the relevant private functions or get the private members, and then you write classes that derive from yours. Note that those will not have direct access as friendship is not inherited, thus the static protected members.
If you use the pImpl idiom, you could make the members of the pImpl itself all public, the pImpl itself private and give your unit tests access to the pImpl (through the same model as above). This is now simpler as you only need to create one method for your "tester".
With regards to data members of a class, in recent years I have been known to put all these into a struct, i.e. have them all public, and then for the class to have a private instance of that struct. It can be easier for handling this kind of thing, and also serialisation / factories to your class, where they can create the struct which is all public, then construct your class from it.
How do I use JUnit to test a class that has internal private methods, fields or nested classes?
It seems bad to change the access modifier for a method just to be able to run a test.
If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.
Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.
Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);
And for fields:
Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);
Notes:
TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for
getDeclaredField.
The setAccessible(true) is required to play around with privates.
The best way to test a private method is via another public method. If this cannot be done, then one of the following conditions is true:
The private method is dead code
There is a design smell near the class that you are testing
The method that you are trying to test should not be private
When I have private methods in a class that are sufficiently complicated that I feel the need to test the private methods directly, that is a code smell: my class is too complicated.
My usual approach to addressing such issues is to tease out a new class that contains the interesting bits. Often, this method and the fields it interacts with, and maybe another method or two can be extracted in to a new class.
The new class exposes these methods as 'public', so they're accessible for unit testing. The new and old classes are now both simpler than the original class, which is great for me (I need to keep things simple, or I get lost!).
Note that I'm not suggesting that people create classes without using their brain! The point here is to use the forces of unit testing to help you find good new classes.
I have used reflection to do this for Java in the past, and in my opinion it was a big mistake.
Strictly speaking, you should not be writing unit tests that directly test private methods. What you should be testing is the public contract that the class has with other objects; you should never directly test an object's internals. If another developer wants to make a small internal change to the class, which doesn't affect the classes public contract, he/she then has to modify your reflection based test to ensure that it works. If you do this repeatedly throughout a project, unit tests then stop being a useful measurement of code health, and start to become a hindrance to development, and an annoyance to the development team.
What I recommend doing instead is using a code coverage tool, such as Cobertura, to ensure that the unit tests you write provide decent coverage of the code in private methods. That way, you indirectly test what the private methods are doing, and maintain a higher level of agility.
From this article: Testing Private Methods with JUnit and SuiteRunner (Bill Venners), you basically have 4 options:
Don't test private methods.
Give the methods package access.
Use a nested test class.
Use reflection.
Generally a unit test is intended to exercise the public interface of a class or unit. Therefore, private methods are implementation detail that you would not expect to test explicitly.
Just two examples of where I would want to test a private method:
Decryption routines - I would not
want to make them visible to anyone to see just for
the sake of testing, else anyone can
use them to decrypt. But they are
intrinsic to the code, complicated,
and need to always work (the obvious exception is reflection which can be used to view even private methods in most cases, when SecurityManager is not configured to prevent this).
Creating an SDK for community
consumption. Here public takes on a
wholly different meaning, since this
is code that the whole world may see
(not just internal to my application). I put
code into private methods if I don't
want the SDK users to see it - I
don't see this as code smell, merely
as how SDK programming works. But of
course I still need to test my
private methods, and they are where
the functionality of my SDK actually
lives.
I understand the idea of only testing the "contract". But I don't see one can advocate actually not testing code—your mileage may vary.
So my trade-off involves complicating the JUnit tests with reflection, rather than compromising my security and SDK.
The private methods are called by a public method, so the inputs to your public methods should also test private methods that are called by those public methods. When a public method fails, then that could be a failure in the private method.
In the Spring Framework you can test private methods using this method:
ReflectionTestUtils.invokeMethod()
For example:
ReflectionTestUtils.invokeMethod(TestClazz, "createTest", "input data");
Another approach I have used is to change a private method to package private or protected then complement it with the #VisibleForTesting annotation of the Google Guava library.
This will tell anybody using this method to take caution and not access it directly even in a package. Also a test class need not be in same package physically, but in the same package under the test folder.
For example, if a method to be tested is in src/main/java/mypackage/MyClass.java then your test call should be placed in src/test/java/mypackage/MyClassTest.java. That way, you got access to the test method in your test class.
To test legacy code with large and quirky classes, it is often very helpful to be able to test the one private (or public) method I'm writing right now.
I use the junitx.util.PrivateAccessor-package for Java. It has lots of helpful one-liners for accessing private methods and private fields.
import junitx.util.PrivateAccessor;
PrivateAccessor.setField(myObjectReference, "myCrucialButHardToReachPrivateField", myNewValue);
PrivateAccessor.invoke(myObjectReference, "privateMethodName", java.lang.Class[] parameterTypes, java.lang.Object[] args);
Having tried Cem Catikkas' solution using reflection for Java, I'd have to say his was a more elegant solution than I have described here. However, if you're looking for an alternative to using reflection, and have access to the source you're testing, this will still be an option.
There is possible merit in testing private methods of a class, particularly with test-driven development, where you would like to design small tests before you write any code.
Creating a test with access to private members and methods can test areas of code which are difficult to target specifically with access only to public methods. If a public method has several steps involved, it can consist of several private methods, which can then be tested individually.
Advantages:
Can test to a finer granularity
Disadvantages:
Test code must reside in the same
file as source code, which can be
more difficult to maintain
Similarly with .class output files, they must remain within the same package as declared in source code
However, if continuous testing requires this method, it may be a signal that the private methods should be extracted, which could be tested in the traditional, public way.
Here is a convoluted example of how this would work:
// Import statements and package declarations
public class ClassToTest
{
private int decrement(int toDecrement) {
toDecrement--;
return toDecrement;
}
// Constructor and the rest of the class
public static class StaticInnerTest extends TestCase
{
public StaticInnerTest(){
super();
}
public void testDecrement(){
int number = 10;
ClassToTest toTest= new ClassToTest();
int decremented = toTest.decrement(number);
assertEquals(9, decremented);
}
public static void main(String[] args) {
junit.textui.TestRunner.run(StaticInnerTest.class);
}
}
}
The inner class would be compiled to ClassToTest$StaticInnerTest.
See also: Java Tip 106: Static inner classes for fun and profit
As others have said... don't test private methods directly. Here are a few thoughts:
Keep all methods small and focused (easy to test, easy to find what is wrong)
Use code coverage tools. I like Cobertura (oh happy day, it looks like a new version is out!)
Run the code coverage on the unit tests. If you see that methods are not fully tested add to the tests to get the coverage up. Aim for 100% code coverage, but realize that you probably won't get it.
If using Spring, ReflectionTestUtils provides some handy tools that help out here with minimal effort. For example, to set up a mock on a private member without being forced to add an undesirable public setter:
ReflectionTestUtils.setField(theClass, "theUnsettableField", theMockObject);
Private methods are consumed by public ones. Otherwise, they're dead code. That's why you test the public method, asserting the expected results of the public method and thereby, the private methods it consumes.
Testing private methods should be tested by debugging before running your unit tests on public methods.
They may also be debugged using test-driven development, debugging your unit tests until all your assertions are met.
I personally believe it is better to create classes using TDD; creating the public method stubs, then generating unit tests with all the assertions defined in advance, so the expected outcome of the method is determined before you code it. This way, you don't go down the wrong path of making the unit test assertions fit the results. Your class is then robust and meets requirements when all your unit tests pass.
If you're trying to test existing code that you're reluctant or unable to change, reflection is a good choice.
If the class's design is still flexible, and you've got a complicated private method that you'd like to test separately, I suggest you pull it out into a separate class and test that class separately. This doesn't have to change the public interface of the original class; it can internally create an instance of the helper class and call the helper method.
If you want to test difficult error conditions coming from the helper method, you can go a step further. Extract an interface from the helper class, add a public getter and setter to the original class to inject the helper class (used through its interface), and then inject a mock version of the helper class into the original class to test how the original class responds to exceptions from the helper. This approach is also helpful if you want to test the original class without also testing the helper class.
Testing private methods breaks the encapsulation of your class because every time you change the internal implementation you break client code (in this case, the tests).
So don't test private methods.
The answer from JUnit.org FAQ page:
But if you must...
If you are using JDK 1.3 or higher, you can use reflection to subvert
the access control mechanism with the aid of the PrivilegedAccessor.
For details on how to use it, read this article.
If you are using JDK 1.6 or higher and you annotate your tests with
#Test, you can use Dp4j to inject reflection in your test methods. For
details on how to use it, see this test script.
P.S. I'm the main contributor to Dp4j. Ask me if you need help. :)
If you want to test private methods of a legacy application where you can't change the code, one option for Java is jMockit, which will allow you to create mocks to an object even when they're private to the class.
PowerMockito is made for this.
Use a Maven dependency:
<dependency>
<groupId>org.powermock</groupId>
<artifactId>powermock-core</artifactId>
<version>2.0.7</version>
<scope>test</scope>
</dependency>
Then you can do
import org.powermock.reflect.Whitebox;
...
MyClass sut = new MyClass();
SomeType rval = Whitebox.invokeMethod(sut, "myPrivateMethod", params, moreParams);
I tend not to test private methods. There lies madness. Personally, I believe you should only test your publicly exposed interfaces (and that includes protected and internal methods).
If you're using JUnit, have a look at junit-addons. It has the ability to ignore the Java security model and access private methods and attributes.
Here is my generic function to test private fields:
protected <F> F getPrivateField(String fieldName, Object obj)
throws NoSuchFieldException, IllegalAccessException {
Field field =
obj.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
return (F)field.get(obj);
}
Please see below for an example;
The following import statement should be added:
import org.powermock.reflect.Whitebox;
Now you can directly pass the object which has the private method, method name to be called, and additional parameters as below.
Whitebox.invokeMethod(obj, "privateMethod", "param1");
I would suggest you refactoring your code a little bit. When you have to start thinking about using reflection or other kind of stuff, for just testing your code, something is going wrong with your code.
You mentioned different types of problems. Let's start with private fields. In case of private fields I would have added a new constructor and injected fields into that. Instead of this:
public class ClassToTest {
private final String first = "first";
private final List<String> second = new ArrayList<>();
...
}
I'd have used this:
public class ClassToTest {
private final String first;
private final List<String> second;
public ClassToTest() {
this("first", new ArrayList<>());
}
public ClassToTest(final String first, final List<String> second) {
this.first = first;
this.second = second;
}
...
}
This won't be a problem even with some legacy code. Old code will be using an empty constructor, and if you ask me, refactored code will look cleaner, and you'll be able to inject necessary values in test without reflection.
Now about private methods. In my personal experience when you have to stub a private method for testing, then that method has nothing to do in that class. A common pattern, in that case, would be to wrap it within an interface, like Callable and then you pass in that interface also in the constructor (with that multiple constructor trick):
public ClassToTest() {
this(...);
}
public ClassToTest(final Callable<T> privateMethodLogic) {
this.privateMethodLogic = privateMethodLogic;
}
Mostly all that I wrote looks like it's a dependency injection pattern. In my personal experience it's really useful while testing, and I think that this kind of code is cleaner and will be easier to maintain. I'd say the same about nested classes. If a nested class contains heavy logic it would be better if you'd moved it as a package private class and have injected it into a class needing it.
There are also several other design patterns which I have used while refactoring and maintaining legacy code, but it all depends on cases of your code to test. Using reflection mostly is not a problem, but when you have an enterprise application which is heavily tested and tests are run before every deployment everything gets really slow (it's just annoying and I don't like that kind of stuff).
There is also setter injection, but I wouldn't recommended using it. I'd better stick with a constructor and initialize everything when it's really necessary, leaving the possibility for injecting necessary dependencies.
A private method is only to be accessed within the same class. So there is no way to test a “private” method of a target class from any test class. A way out is that you can perform unit testing manually or can change your method from “private” to “protected”.
And then a protected method can only be accessed within the same package where the class is defined. So, testing a protected method of a target class means we need to define your test class in the same package as the target class.
If all the above does not suits your requirement, use the reflection way to access the private method.
As many above have suggested, a good way is to test them via your public interfaces.
If you do this, it's a good idea to use a code coverage tool (like EMMA) to see if your private methods are in fact being executed from your tests.
Today, I pushed a Java library to help testing private methods and fields. It has been designed with Android in mind, but it can really be used for any Java project.
If you got some code with private methods or fields or constructors, you can use BoundBox. It does exactly what you are looking for.
Here below is an example of a test that accesses two private fields of an Android activity to test it:
#UiThreadTest
public void testCompute() {
// Given
boundBoxOfMainActivity = new BoundBoxOfMainActivity(getActivity());
// When
boundBoxOfMainActivity.boundBox_getButtonMain().performClick();
// Then
assertEquals("42", boundBoxOfMainActivity.boundBox_getTextViewMain().getText());
}
BoundBox makes it easy to test private/protected fields, methods and constructors. You can even access stuff that is hidden by inheritance. Indeed, BoundBox breaks encapsulation. It will give you access to all that through reflection, but everything is checked at compile time.
It is ideal for testing some legacy code. Use it carefully. ;)
First, I'll throw this question out: Why do your private members need isolated testing? Are they that complex, providing such complicated behaviors as to require testing apart from the public surface? It's unit testing, not 'line-of-code' testing. Don't sweat the small stuff.
If they are that big, big enough that these private members are each a 'unit' large in complexity—consider refactoring such private members out of this class.
If refactoring is inappropriate or infeasible, can you use the strategy pattern to replace access to these private member functions / member classes when under unit test? Under unit test, the strategy would provide added validation, but in release builds it would be simple passthrough.
I want to share a rule I have about testing which particularly is related to this topic:
I think that you should never adapt production code in order to
indulge easer writing of tests.
There are a few suggestions in other posts saying you should adapt the original class in order to test a private method - please red this warning first.
If we change the accessibility of a method/field to package private or protected, just in order to have it accessible to tests, then we defeat the purpose of existence of private access directive.
Why should we have private fields/methods/classes at all when we want to have test-driven development? Should we declare everything as package private, or even public then, so we can test without any effort?—I don't think so.
From another point of view: Tests should not burden performance and execution of the production application.
If we change production code just for the sake of easier testing, that may burden performance and the execution of the application in some way.
If someone starts to change private access to package private, then a developer may eventually come up to other "ingenious ideas" about adding even more code to the original class. This would make additional noise to readability and can burden the performance of the application.
With changing of a private access to some less restrictive, we are opening the possibility to a developer for misusing the new situation in the future development of the application. Instead of enforcing him/her to develop in the proper way, we are tempting him/her with new possibilities and giving him ability to make wrong choices in the future.
Of course there might be a few exceptions to this rule, but with clear understanding, what is the rule and what is the exception? We need to be absolutely sure we know why that kind of exception is introduced.
I am trying to program according to Behavior Driven Development,
which states that no line of code should be written without writing
failing unit test first.
My questions:
how to use BDD with private methods, and
how can I unit test private methods?
Is there better solution than:
making private methods public first and then making them private
when I write public method that uses those private methods;
or
in C# making all private methods internal and using InternalsVisibleTo
attribute.
When you write code test-first, you write against the public interface. There are no private methods at this point.
Then you write the code to pass the test. If any of that code gets factored into a private method, that's not important -- it should still be there only because it is used by the public interface.
If the code isn't written test first, then -- in .net, anyway -- reflection can be used to directly prod private methods; though this is a technique of last resort.
Private methods are internal implementation details. They should not be tested directly, as they will be tested indirectly via testing your public interface. If for some reason a private method is not covered when your public interface is fully tested, then the private method is not required, and it should be removed.
Generally, it is a bad idea to bind test code to private implementation details. That couples your test to those private details, reducing your freedom to change those details at will, even if they don't affect the publicly facing interface and behavior. That increases the amount of effort required to write and maintain your unit tests, which is a negative thing. You should strive for as much coverage as possible, while only binding to the public interface.
Short answer: You don't test private methods.
If you have programmed well, the code coverage of your tests should be testing private methods implicitly.
If a private method method exists, it's there to be used by a public method. Therefore I'd write a test for the public method.
I write my tests to test the public parts of a class. If the class is well designed then the private parts get tested by default.
If the private method isn't called from a public method, then why does it exist?
In your case I'd do the following
* Write failing test for the public method
* Write public method that calls the private method that doesn't exist yet(test still fails as your class is incomplete
* Write the private method
* Test should now pass
Short answer: You can't test a private method.
Long answer: You can't test a private method, but if you're inclined to test whatever it does consider refactoring your code. There are two trivial approaches:
Test the public method that accesses the private method.
Extract the private code to its own class, i.e. move the implementation so it can become appropriately public.
The first one is simple but has a tendency to let you shoot your own foot as you write more tests and the latter promotes better code and test design.
Contrived answer: Okay, so I lied. You can test a private method with the help of some reflection magic (some TDD tools support testing private methods). In my experience though, it leads to convoluted unit tests. Convoluted unit tests leads to worse code. Worse code leads to anger. Anger leads to hate. Hate leads to suffering…
The direct effect of production code becoming worse is that the class under test tend to become large and handles many things (violation of Single Responsibility Principle) and harder to maintain. This defeats the purpose of TDD, that is to get production code testable, extensible and more importantly: reusable.
If you're writing tests for a class that is deployed, you could investigate everything that calls the private method and write tests accordingly. If you have the chance to rewrite the class then please do refactor it by splitting the class up. If you're lucky then you'll end up with some code reuse that you can utilize.
We can Unit Test static and instance private methods using PrivateType and PrivateObject respectively. The following 2 articles explains these techniques
1. Unit Test Private Static Method in C#.NET
2. Unit Test Private Instance Method in C#.NET
I agree with the point that has been made about not testing private methods per se and that tests should be written against the public API, but there is another option you haven't listed above.
You could make the methods protected then derive from the class under test. You can expose the base protected method with a public method on the derived class, for example,
public class TestableClassToTest : ClassToTest
{
public new void MethodToTest()
{
base.MethodToTest();
}
}
You might be using this Extract and Override pattern already to override virtual properties of the base class for dependency injection, in which case this may be a viable option for you.
Mbunit Reflector helps you with this.
Reflector objectReflection = new Reflector(new ObjectWithprivateMethods());
objectReflection.InvokeMethod(AccessModifier.NonPublic,,"Add",1,6));
A blog post about it.
I've been fighting with it for over 1 month, but found the answer:
var objectOfPrivateMethod = new ObjectOfPrivateMethod(); //yes here is contructor
object[] arguments = { }; // here as Object you provide arguments
var extractedPrivateMethod = typeof(ObjectOfPrivateMethod).GetMethod("Name_Of_Private_Method", BindingFlags.NonPublic|BindingFlags.Static); //if fails returns null. delete flag static if it's not static. Returns your method as an object.
Assert.AreNotEqual(null, extractedPrivateMethod, "Mathod does not exist"); // good to catch if even exists.
object result = extractedPrivateMethod.Invoke(null, arguments); // here as object you'll get return value of your function. change null for object of class where is method, if your method is not static
that's all.
You should only be testing the external API of your classes, i.e. the public methods. If your tests aren't hitting code in the private methods then either you need to write more tests or refactor the class.
The whole point of testing an API, especially one that will be distributed to third parties, is that you can change the internal structure of the class as much as you want, as long as you don't break the external contract of it's public methods.
As you've identified, this is where BDD comes into play over 'traditional' TDD using mock classes, where every method call has to be set-up in advance for the test. I'm not an expert on either of these, hopefully someone else can answer that one better than I can.
If you find yourself wanting to test a private method then there is something complex in it and you are probably right to want to test it, this is a design smell. Exposing the method on the interface just swaps one smell for another worse one.
Time to refactor :)
Usually I factor out the inner complexity into a helper class. However check the method for 'Feature Envy' or 'Inappropriate Intimacy'. There may be a better place for the method to live. With Extension methods in .net now, even base types could be a good candidate.
Good Luck
If you really believe that a private method is complex enough that it deserves unit tests of it's own - it's an indicator that your class is doing too much and you should extract part or all of that private method into a class of its own behind an interface.
Mock the interface when testing the original class. You should now have a public accessor to the new class which was previously the private method.
Sometimes when dealing with old code that was either poorly written or not written using TDD there may be a need to test the private classes. In this case you should use reflection, but where possible update the code to follow closer to the TDD approach.