Some questions that will guide you to the best solution:

Is the file a known length?

Does the file really contain binary data (of which text is certainly a subset) or is it only ever containing ASCII characters (i..e text)? By the fact that you want to populate a text box with the byte array I am assuming simple ASCII data here

No matter what the answers are, there is a way to open the file and read it without having to read the bytes one at a time.

regards
Paul Lewis