C#:Active Directory:Uncheck User cannot change password

        public void SetUserCanChangePasswordsPropertyUncheck()
        {
            DirectoryEntry de = GetDirectoryObject(UserName);
            string PASSWORD_GUID = "{ab721a53-1e2f-11d0-9819-00aa0040529b}";
            ActiveDs.IADsSecurityDescriptor sd =
                (ActiveDs.IADsSecurityDescriptor)de.Properties["ntSecurityDescriptor"].Value;
            ActiveDs.IADsAccessControlList acl = (ActiveDs.IADsAccessControlList)sd.DiscretionaryAcl;
            //ActiveDs.AccessControlEntry ace = new ActiveDs.AccessControlEntry();
            ActiveDs.ADS_ACETYPE_ENUM aceType;

            //look for existing ace and get rid of
            foreach (ActiveDs.AccessControlEntry ace in acl)
            {
                if (!(ace.ObjectType == null) && ace.ObjectType.ToLower() == PASSWORD_GUID)
                {
                    if (ace.Trustee == "Everyone")
                    {
                        acl.RemoveAce(ace);
                        de.CommitChanges();
                    }
                    else if (ace.Trustee == "NT AUTHORITY\\SELF")
                    {
                        acl.RemoveAce(ace);
                        de.CommitChanges();
                    }
                }
            }

            //now put in the one we want                
            sd.DiscretionaryAcl = acl;
            de.Properties["ntSecurityDescriptor"].Value = sd;
            de.CommitChanges();
        }

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