how can change active directory password

add New user successfully

      public static void AddUser(ADUser adUser)

        {

            // Local variables

            DirectoryEntry oDE = null;

            DirectoryEntry oDENewUser = null;

            DirectoryEntries oDEs = null;

            try

            {

                oDE = GetDirectoryEntry(GetADPath(PROD, adUser.UserType));

                // 1. Create user account

                oDEs = oDE.Children;

                oDENewUser = oDEs.Add("CN=" + adUser.UserName, "user");

                // 2. Set properties

                SetProperty(oDENewUser, "givenName", adUser.FirstName);

                SetProperty(oDENewUser, "sn", adUser.LastName);

                SetProperty(oDENewUser, "mail", adUser.Email);

                SetProperty(oDENewUser, "sAMAccountName", adUser.UserName);

                oDENewUser.CommitChanges();

                /// 4. Enable account           

                EnableAccount(oDENewUser);

                // 3. Set password

                //SetPassword(oDENewUser, adUser.Password);

                SetPassword1(oDENewUser.Path, adUser.Password);

                oDENewUser.CommitChanges();

                oDENewUser.Close();

                oDE.Close();

            }

            catch (Exception ex)

            {

                throw ex;

            }
        }
I have try the following 2 SetPassword methods but getting error.
Method 1.

     internal static void SetPassword1(string path, string userPassword)

        {

            //Local variables

            DirectoryEntry usr = null;

            try

            {

                usr = new DirectoryEntry();

                usr.Path = path;

                usr.AuthenticationType = AuthenticationTypes.Secure;

                object ret = usr.Invoke("SetPassword", userPassword);

                usr.CommitChanges();

                usr.Close();

            }

            catch (Exception ex)

            {

                throw ex;

            }

        }

The exception raised (Error Code 80072035: The server is unwilling to process the request)

internal static void SetPassword(DirectoryEntry de, string userPassword)

        {

            //Local variables

            //DirectoryEntry usr = null;

            string quotePwd;

            byte[] pwdBin;

            try

            {

                quotePwd = String.Format(@"""{0}""", userPassword);

                pwdBin = System.Text.Encoding.Unicode.GetBytes(quotePwd);

                de.Properties["unicodePwd"].Value = pwdBin;

                de.CommitChanges();

                //usr.Close();

            }

            catch (Exception ex)

            {

                throw ex;

            }

        }
The exception raised ("Exception has been thrown by the target of an invocation.")
Is there an easy way to tell if there is a problem with changing a password?
Please reply me as soon as possible.
Thanks.