Hi Soon,
For the purpose of testing you have a few alternatives which you may or may not like:
1. call the private method through reflection - a bit cumbersome but once you get the syntax down it gets easy
2. make the method internal, and make it accessible to the test class by using InternalsVisibleTo in Assembly.cs. This of course has the impact of making this method accessible to any other class in the same assembly - something you may not want.
3. Create an internal wrapper method that calls the private method - again, you may not want one.
Basically the most "correct" way to do it is through reflection, but it's a bit unwieldy. "Correct" and hard is a deterrent for me, so I usually go with one of the other ways.
Doron