Format a Social Security Number (SSN) as XXX-XX-XXXX from XXXXXXXXX

For a simple, short, and self commenting solution, try:

String.Format("{0:000-00-0000}", 123456789) 

123456789 representing your SSN variable.

Check out the String.Insert method.

string formattedSSN = unformattedSSN.Insert(5, "-").Insert(3, "-");