Generating a character serial AAAA, AAAB, AAAC

Question

Hi All, I hope all is well. i have been trying to generate a unique identity code for each data record and i trying to use characters for example AAAA. AAAB, AAAC.

Now i move to the last record and the record appears in textbox1 as "AAAC" , but i would like to generate the next unique identity code character "AAAD" into another textbox2. how can i get about this.
Any help will be greatly appreciated. I using c sharp.
thanks