Help with casting to a different class (upcasting?)

As long as myFunction is creating an instance of MyTable and MyTable is a class that derives from DataTable it works.  Can you post the relevant lines from myFunction?

My code seems pretty straighforward. Here is a watered-down version that sl does not work: It get an Invalid Cast exception at the line with the ***

public void main
[  
   ProfileList.ListDataTable ldt = EEExample();
}

public ProfileList.ListDataTable EEExample()
{
   return MyFunction();
}

private ProfileList.ListDataTable MyFunction()
{
   DataTable dt = new DataTable();
   return (ProfileList.ListDataTable)dt; //***
}

How can I cast the System.Data.DataTable  to my DataTable-derived class type at the line above (with the ***)?

Thank you

You need to actually construct an instance of ListDataTable somewhere, most likely in MyFunction.  Probably something like this:

private ProfileList.ListDataTable MyFunction()
{
   DataTable dt = new DataTable();
   ListDataTable oLdt = new ListDataTable( dt );
   return oLdt;
}

And your ListDataTable class needs a constructor (i.e., a conversion operator) that accepts a DataTable.

I'm sorry, bu I don't see how this helps.

Imagine (as my first example in the first message shows) that I need to call a "black box" funtion that returns a System.Data.DataTable object. I want to "import" that object instance into my ProfileList.ListDataTable data type--something I felt I should easily be able to do since my ProfileList.ListDataTable is a classed derived from System.Data.DataTable. That's why I though it should be easy to cast any System.Data.DataTable object instace to a data type of ProfileList.ListDataTable.

What is wrong with that logic?

I was trying to go by your latest code.  Your MyFunction can't return an instance of a ListDataTable unless you create one.  With this inheritance, you can treat a ListDataTable as a DataTable but not the other way around.

Your logic description falls apart when you say cast any DataTable into a ListDataTable.  A ListDataTable is a derived class, so this kind of cast is a downcast.  Meaning, it fails unless the object being cast is actually a ListDataTable.  If you want to create an instance of a ListDataTable from the DataTable, you provide a conversion operator - a constructor for ListDataTable that takes in a DataTable and constructs a new ListDataTable.

OK--I suppose that I was always upcasting and not downcasting. So "downcasting" is new to me.

Are there any external resources (links) that show how to do this in practice? Are some approaches better than others? Is "downcast" the right way to describe this or are there other terms that are more widely used?

Thank you.

Just one more question along the lines of the original:

If you have a framework object of type FrameWorkClass and you want to enhance objects of that type, I thought you can just use that class as a base class for a new class. For example:

class MyClass : FrameWorkClass

Now, I thought I could just create new objects like:
  MyClass oMyClass = new MyClass()

The problem is what do you do when the FrameWorkClass object already exists and you want to "import" all of its data to your newn oMyClass object???

I just don't know what doing that is called and since it's so new to me, I would appreciate a link to a tutorial or other resource that can discuss this further.

Is this not a good or common thing to be doing? Should I be taking another approach to "downcase" my object?

Thanks again!

I meant "downcast" (not "downcase") my object, above.

I still need help. This question is still open. I'm just not getting what I should or shouldn't be doing to successfully downcast a derived object instance...

I reviewed my code.
It seems that there is no easy way to downcast unless I use operator overloads or other undesirable strategies.

I have instead changed the idea of inheriting from DataTable. Now, I am using a public property called ListDataTable which is of type DataTable. When I have an object instance of my class, I will refer to its DataTable like this:

oMyClass.ListDataTable

Casting should no longer be required because ListDataTable is indeed a Sql.Data.DataTable object instance.

Thank you for your feedback.

This issue is now closed.