This requires reference types that can be null to be annotated by a ? operator, similar to value types.
This gives the advantage that the compiler can warn against any null dereference exceptions, of which this commits elimits a few.
To make the underlying protocol implementation not any more complicated and to eliminate existing problems, and not that precise error reporting, I replaced some return null statements with explicit Exceptions. This lead to the assumption that those core protocoll functions always return non-null objects if they do not throw, making the PLC code simpler.
Adjust some NotConnected tests to look for explicit PlcException instead of NullReferenceException.
* Add StringEx.ToByteArray(...)
* Add Serialization.SerializeDataItem(DataItem)
Supports StringEx VarType or offloads to SerializeValue method.
* Use SerializeDataItem in S7WriteMultiple
* Assume string length without header in StringEx.ToByteArray
VarTypeToByteLength already assumed that StringEx declared count for
the number of characters without the header, this now matches that
behavior.
* Add unit tests for StringEx conversions
* Fix incorrect value passed to Encoding.GetBytes
The length must actually be within string limits.
