Update value Declaration attribute for null or empty get errror?

Here's the code to update mail, department, header and mobile to zero or empty AD user value.

            var adSearch = new DirectorySearcher(ConnectHelper.ContxEntry);
            adSearch.Filter = "samAccountName=" + "....";
            var result = adSearch.FindOne();
            if (result != null)
            {
                DirectoryEntry user = result.GetDirectoryEntry();
                user.Properties["mail"].Value = null;
                user.Properties["Department"].Value = null;
                user.Properties["Title"].Value = null;
                user.Properties["mobile"].Value = null;
                user.CommitChanges();
            }

      

        
        
        
      

    

Error: "The syntax for the attribute specified for the directory service is invalid."

How can I set the value of some properties to null or empty?