I know that I should provide all necessary info for the function and i am able to do unit test comfortably. but under complex circumstance i don't know how, could you please shed light on it?

My app contains three projects with in a solution,one is for web site, one is a class library project for business logic tier and the other is also class library project for data access tier.

I paste my code here:

Business Logic Tier:

PO.cs

public class PO

{

 private PODao dao=new PODao();

 public bool InsertPO(POEntity)

 {

   ... //some validation logic

   return dao.InsertPO(POEntity);

 }

}

DataAccess Tier:

PODao.cs

public class PODao

{

  public bool InsertPO(POEntity)

 {

   //use Enterprise Lib to execute database operation

   Database db = DatabaseFactory.CreateDatabase();

   ... //construct DbCommand

  try{

   db.ExecuteNonQuery(cmd);

   return true;

  }

  catch{

   //exception logic puts here

   return false;

  }

 }

}

Asp.net presentation tier:

in the button click event handler on my POSubmit.aspx page,I wrote some codes like the followings:

button_click()

{

  POEntity entity=new POEntity();

  ....assign POEntity property value from web controls value

 PO po=new PO();

 po.InsertPO(entity);

}

and I put the connection string info in the web.config file.

Now How can I do unit test for this InsertPO method of PO class in the business logic tier? My steps is as followings:

first right click on the "InsertPO" method name within VSTS IDE, this will generate plumbing unit test codes as followings for this method

        /// <summary>
        ///A test for InsertPO (POEntity)
        ///</summary>
        [TestMethod()]
        public void InsertPOTest()
        {
            PO target = new PO();

            POEntity po = null; // TODO: Initialize to an appropriate value

            bool expected = true;
            bool actual = false;

            actual = target.InsertPO(po);

            Assert.AreEqual(expected, actual, "PO.InsertPO(POEntity) did not return the expected value.");
            //Assert.Inconclusive("Verify the correctness of this test method.");
        }

As you said, I need to provide each information for the function i am testing, I was able to use some fake data to mock POEntity,but this unit test will fail when executing, I think it is that the code highlighted with red color(Database db = DatabaseFactory.CreateDatabase();) caused this unit test fail, because this code will use the connection string info I stored in web.config file to create a instance of Database class, howerver, when executing unit test, it can't get this connection string info. How should I provide such kind of information for my unit test? Actually when doing unit test, I always puzzled how to do with such circumstance. google for unit test, there are a lot of samples demonstrating how to do unit test for a method whose function is calculate sum of two addend and alike, but those samples was not so helpful I think.