How to get a different type of value from a specific implementation if only the interface / abstract class is known?

Question

What I use:
VB.NET, NET 3.5, OpenXML SDK 2.0

What I want to do:
I am creating an xlsx reader / writer for my application (based on the OpenXML SDK 2.0). I want to read xlsx files and store the data contained in each line in DTO / PONO. Next, I want to read the xlsx file, and then change it and save.

my thoughts:
Now my problem is not in the OpenXML SDK, I can do what I need.

My problem is how to structure my components. In particular, I have problems with polymorphism at the lowest level of a table, cell.

A cell in Excel / OpenXML can have different types of data associated with it. Like time, date, number, text or formula. These different types need to be handled differently when reading / writing from / to a spreadsheet.

I decided to have a common interface for all subtypes like TextCell, NumberCell, DateCell, etc.

Now, when I read a cell from a spreadsheet, the / Factory method can decide what type of cell will be created.

Now, since the cell is abstract from the actual implementation, it does not know / does not need what type it is. To write / modify a cell, I solve this problem by calling .write (ICellWriter) in the cell I want to save. Since the cell itself knows what type of data it contains, it knows which ICellWriter method it needs to call (static polymorphism).

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Edit:

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Answer 1

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