Set value of file field using entity_metadata_wrapper non working

Question

I have a custom entity with a file field accepting multiple values. I would like to set file values according to https://www.drupal.org/docs/7/api/entity-api/entity-metadata-wrappers#example-value-set-save but none of the two methods are working here. I always get this error

error  php  Error: Call to a member function set() on null in include().

I am running Drupal 7, file_entity module enabled. Here follows the code:

$file = file_load(8172);
$wrapper = entity_metadata_wrapper('sys_ind', 13718);
$wrapper->field_allegati->file->set( $file );
$wrapper->save();

Answer 1

Fixed! Thanks to this post Save a new value with entity_metadata_wrapper to an entity field which is an array

$file = file_load(8172);
$node = entity_load('sys_ind', array(13718));
$wrapper = entity_metadata_wrapper('sys_ind', '13718');

$wrapper->field_allegati[NULL]->file->set( $file ); # This worked

$wrapper->save();

so the fix is using NULL (at least in multivalue field like mine is), not 0 or any other index nor [].

Answer 2

Try this to see if it works:

$wrapper->field_allegati[0] = (array) $file;

The wrapper seems to struggle with file fields