How to generate strings with pattern?

actually I tried this one:

s = WebConfigurationManager.ConnectionStrings["AmicassaConnectionString2"].ConnectionString;
amicassa = new SqlConnection(s);
amicassa.Open();
SqlCommand cmdd = new SqlCommand("SELECT Milestone_Code FROM tblMilestone WHERE Milestone_Name = '" + dplmilestone_checklist.SelectedItem + "'", amicassa);
SqlDataReader drr = cmdd.ExecuteReader();
if (drr.HasRows)
{
while (drr.Read())
{
string aa = drr[0].ToString();
string[] parts = aa.Split('-');
int number = Int32.Parse(parts[1]) + 5;
string result = parts[0] + "-CC-" + number.ToString();
txtchecklist_code.Text = result;
}
}
drr.Close();
amicassa.Close();

but if I am going to add another sub item for the same milestone_code and save it, they' re both the same which should not be.

PPY-05---> PPY-CC-10
---> PPY-CC-10
It should not be like that.

How to do it?

My answers:

1. the number of sub items will be based from the user input. but the user will not input the number of sub items. the system will just automatically create their codes when the user would like to add another record on that milestone.

2. I need to get the milestone_code from the database since the dropdownlist will display the milestone_name of each code. That's part of the project requirements.

3 and 4. Its only CC for check_code and CC-AC is for activity_code. They will be determined by this flow: at first page, the system will display a dropdownlist that contains the milestone_name. below this dropdownlist is a textbox for the check_code. if the user selects a milestone_name(for example is BOOKING), the corresponding milestone_code(which is BOO-05) of that milestone_name will be get to automatically create the check_code in the pattern of this,"BOO-CC-05" and then a save button to store the check_code. if the user wants to add another check_code from the same milestone_name: BOOKING, the check_code should be now like this:"BOO-CC-10". meaning, the system should validate if the check_code is already existing. if it is, then it will add another 5 to it. Likewise, a page that has the same setup will be for the generation of activity_code. same set up and process like the one I explained above. But there will be 2 dropdownlist for now. the first one is for the milestone_name and the other one is for the checklist _name. If I select a milestone_name, the second dropdownlist will display only the checklist_name that is related to that milestone_name. The check_code of that corresponding checklist_name will be get and will use for the generation of activity_code that will be placed in a textbox in a pattern like this:"BOO-CC-AC-05". Again, if the user wants to add another record, the system will validate if the activity_code exist and if yes, it will add another 5 to it.