The solution obove does not work for me but I get another solution:
TaxonomyField managedMetadataField = item.Fields[fieldGuid] as TaxonomyField; TaxonomyFieldValueCollection managedMetadataFieldValue = (TaxonomyFieldValueCollection)managedMetadataField.GetFieldValue("");
managedMetadataField.SetFieldValue(item, managedMetadataFieldValue);
TaxonomyField managedMetadataField = item.Fields[fieldGuid] as TaxonomyField;
TaxonomyFieldValueCollection managedMetadataFieldValue =
(TaxonomyFieldValueCollection)managedMetadataField.GetFieldValue("");
managedMetadataField.SetFieldValue(item, managedMetadataFieldValue);