recepkirman:
C# side works well . I guess, my issue from arduino side. How do I taking barcode output from thermal printer like code128 or similar (which one is best) for character values.
I don't understand this at all.
How is the printer sending barcode values? I thought you wanted to print the barcode?
I have no idea what code128 means
Is the printer connected to the Arduino or to the PC?
If it is connected to the Arduino have you tried a simple program to print stuff without taking input from the PC?
Have a look at Serial Input Basics
Using the String (capital S) class can cause problems in the small memory of an Arduino. Use C strings (small s) which are char[] arrays terminated by 0.
...R