How To Change a User Password with C# and Active Directory

using System;
using System.DirectoryServices;

class Testclass
{
        static void Main()
        {
                string userName = "Bob";
                string oldPassword = "123shoot"
                string newPassword = "KJ#$#H";

                Console.WriteLine("changing password for " + userName + " from "  
                                        + oldPassword + " to " + newPassword);

                ChangePassword(userName, oldPassword, newPassword);

        }

        public static void ChangePassword(string userName, string oldPassword, string newPassword)
        {
                string path = "LDAP://CN=" + userName + ",CN=Users,DC=demo,DC=domain,DC=com";

                //Instantiate a new DirectoryEntry using an administrator uid/pwd
                //In real life, you'd store the admin uid/pwd  elsewhere
                DirectoryEntry directoryEntry = new DirectoryEntry(path, "administrator", "password");

                try
                {
                   directoryEntry.Invoke("ChangePassword", new object[]{oldPassword, newPassword});
                }
                catch (Exception ex)  //TODO: catch a specific exception ! :)
                {
                   Console.WriteLine(ex.Message);
                }

                Console.WriteLine("success");
        }
}


http://www.rootsilver.com/2007/08/how-to-change-a-user-password.html