Reading Data From Serial Port For Barcode Scanner

I am developing a Winforms application which read barcode from barcode scanner. It's an Human wedge scanner. This will work if I placed the cursor in any textbox or notepad. But i don't want like that. Reason is that it's cordless scanner. If some want to scan some thing in store they will move around, so there is possibility of screen saver coming or system gets locked if that happens the barcodes wont capture, but the user who is scanning the barcodes won't know this thing.

So i am reading the data using serial port communication and my scanner sending the data character by character, Not a whole string. so for that I wrote the following code. Thsi code will works if i am writing to any textbox. But it is not working if want to store it in any global string. Only first character is storing in the string. The below code is working perfectly fine. but if want to save the string in Global String variable instaed of textbox the only first character in the string is storing.

[Code]...