As Lesiok said... search and read.

The SerialPort is in no way guaranteed to send all data at once... no matter what the size of the data. It is up to the programmer to make sure YOU know how much data is being sent and/or what marks the beginning and end of the data.

If you truly have no way to know how much data is being transmitted, or what the beginning and end of the data is... then how can you possibly know WHAT the data is?