casting - Cast child as far parent and then as direct child c# -

i can’t cast parent class child – keep getting invalid cast exception.

i can’t overcome problem , rally use help.

i’m developing application creates documents. there total of 20 documents. 19 of themare unique , 1 derives different 1 extending contents. certificates have common header information , document-specific body.

so created document class. never instantiate document class directly, through children. 19 documents directly derived document class. document class defines fields , properties common documents along methods access sql database. constructors of each document (child), when initiated no argument call document’s (parent) constructor passing integer helps constructor select unique information common database table, such unique title of document, disclaimer or footer note.

each document created win form interface. created base interface common certificates , operates on header information contains methods common document-windowsform interactions. each child document has own form derives parent form. on initialization each form instantiates corresponding document (child) class , displays document title, disclaimer , footnote on load. since documents rely on header info base form methods defined inside base form. methods accept , return document data type (parent) because never know form call them. each time pass newly instantiated document document class (parent) , cast returned datatype match needed. way retain unique properties od child documents while using generic methods collect common information. works 19 of 20 documents. 20th problem. doc20 derives doc19 derives document class. likewise form20 derives form19 derives genericform. doc20 in fact significant extension of doc19.

on form init instantiate new doc20 object. grab title, disclaimer , footnote on form load event , display needed.

then try call method in genericform accepts , returns document class , cast return value doc20. works.

next call method in form19. method accepts document class , returns document class , tasked collecting data controls common form19 , form20. pass doc20 object document. inside method need access properties specific doc19 (thus doc20) try cast doc19 , exception gets thrown. interestingly intelisense has no problems syntax , application compiles fine. idea why cant cast parent child?