vb.net cast parent class object as child class object

Hello all,

I hope someone can help me!!

I have a set of classes in my ClassLibrary that I am creating in VB.NET. Each class represents one row of data in a db table which means these objects need to be dynamically created.

Since there is one class definition for each table, there are some methods in these classes that are general like update this record or delete this record.

So i created a Parent class called MDALObject that has these methods.

So now when i query the db and get the records, I iterate over the records and create the MDALObject class objects and return a Collection of them to the application.

My application tries to cast that MDALObject which is a parent class to a child class object but i get this error:

Specified cast is not valid.

I figured that since the child object inherits from the MDALObject, I should be able to cast the parent class object as the child class.

What am I doing wrong?

The code in the app looks like this:

Dim apvlf As New ActivePatientVisitListFactory
Dim apv As ActivePatientVisit 'Child class of MDALObject

apv = apvlf.getByPK(1) 'This is where the error occurs

Response.Write(apv.GetType())

I have also tried using CType() as:

apv = CType(apvlf.getByPK(1), ActivePatientVisit)

This doesn't work either.

Any help would be appreciated!