Get datastores names with searchManagedEntity function

hi,
I'm trying to locate a specific nas storage 

Datastore datastore = (Datastore) new InventoryNavigator(rootFolder).searchManagedEntity("DataStore", "nfs_store");

System.out.print(datastore.getName());

Unfortunetaly, it does not return any name. It seems that the datastore object still null. By the way, I test the same with host as following:

HostSystem host = (HostSystem) searchManagedEntity("HostSystem", "host1");

System.out.print(host.getName());

and I got the right name.

Could be the searchManagedEntity function is not the suitable one to search datastore managed objects? if it is the case how is it possible to get datastores properties ?

Note: searchManagedEntity takes two arguments for the managed object: searchManagedEntity(type,name)

Thanks in advance.