I want to write PLC programs in Ladder diagrams so I tought of creating a program using C#, which would allow me build my ladder programm
that is the difficult part.

First of all WHY? you can buy small PLCs for less than $100
second BEWARE industrial PLC can take an awful lot of abuse on the inputs and outputs, THAT is where the cost comes from. If your thingy can not handle e.g. an 'accidental' 220V on an input you will drown in warranty repair.

so that the C# will assign a number of bits (lets say 8 bits) and when these bit as received by the micro, the C programm will interpret those 8 bits as for exmple:
that will be way too limited, use 'control blocks' (e.g. 8 bytes)

Erik