How to Return a string representing the hexadecimal value of a number dispalyed in a textbox

Question

Hello All,
i am writing an application in C# which communicates with serial port by sending hex data to the port. how can i convert a textbox.text contents into Hex values of the number displayed on the textbox.text (displays these data after computation).
example: my textbox.text may display any of these values like 16,17,24,34,39,49,52,65,68,81,89,99,101 or 129. is there any method to convert the displayed values on the textbox to hexadecimal equivalent of the value on the textbox and write such to the serilport.

any code/ clue will be welcomed.