Loop C#

Question

char [ ] letter = new char {‘A’ ,’B’,’C’,’D’,’E’......};
char [] number =new string {‘0’,’1’,’2’,’3’,’4’,......};

char[] chAll= Textbox.Text.ToCharArray();

How to loop this ?
When we have 2 characters Array Letter and Number above ?

When I put like this 123 ; 2AB; B2.....
Ex1: 123
000
001
....
333

Ex2: 2AB
000
001
002
00A
00A
00B
.....
BBB