string a = "Welcome";
string b = null;
int num = a.Length;
int ascii;
char ch;
b = a.Substring(a.Length - 1, 1);
for (int i = 0; i =(int)'a';i--){char old1=(char)i; char new1= (char)(i+1);str = str.Replace(old1,new1);}for(int i=(int)'Z';i>=(int)'A';i--){char old1=(char)i; char new1=(char)(i+1); str = str.Replace(old1,new1);}Console.WriteLine(str.ToString());大概就这样了，偷懒没考虑没考虑吧A—》Z or a—》z
