方法一：

            string str = 123456789;

            str = int.Parse(str).ToString("##-###-####");

方法二：

            string str = "123456789";

            str = str.Insert(5,"-");

            str = str.Insert(2, "-");

方法三：

首先引入命名空间：using System.Text.RegularExpressions

            //Regex reg = new Regex(@"^(\d{2})(\d{3})(\d{4})$");

或者

            Regex reg = new Regex(@"^(\d{2})(\d{3})(\d{4})$");

            str = reg.Replace(str,"$1-$2-$3");