c# - list.AddRange Cannot implicitly convert type 'void' -

   namespace subscriptionwebsite.placeholders {     public class treedata     {         public int id { get; set; }         public int? parentlocationid { get; set; }         public string name { get; set; }         public int? locationlevel { get; set; }         public string participation { get; set; }     } }   namespace subscriptionwebsite.managers {     public class treedatamanager     {         subscriptionwebsite.entities.acc_core_dbentities db = new subscriptionwebsite.entities.acc_core_dbentities();          public list<treedata> gettreedata()         {             list<treedata> location = db.frm_location.where(x => !x.removed && x.revision == 0).select(loc => new treedata                {                    id = loc.id,                    parentlocationid = loc.parentlocationid,                    name = loc.frm_location_level.systemid.equals(5) ? loc.frm_location_address.firstordefault(c => !c.removed && c.revision == 0).street1 : loc.name,                    locationlevel = loc.frm_location_level.systemid,                    participation = loc.isactive ? "yes" : "no",                }).tolist();              list<treedata> meters = db.frm_connection_meter.where(x => !x.removed && x.revision == 0).select(l => new treedata              {                  locationlevel = 6,                  id = l.id,                  parentlocationid = l.frm_location.id,                  name = l.meternumber,                  participation = l.ismain ? "yes" : "no",// change isactive after db update              }).tolist();               return location.addrange(meters));         }     } } 

if try put 2 lists of treedata

return location.addrange(meters)); 

i following error: cannot implicitly convert type 'void' system.collections.generic.list

i know return type of the.addrange void(null) how can put 2 list ?

list.addrange doesn't return since modifies list directly:

location.addrange(meters); return location; 

if don't want modify can use linq:

return location.concat(meters).tolist(); 

but wouldn't create other 2 lists, more efficient:

public list<treedata> gettreedata() {     var locations = db.frm_location         .where(x => !x.removed && x.revision == 0)         .select(loc => new treedata         {             id = loc.id,             parentlocationid = loc.parentlocationid,             name = loc.frm_location_level.systemid.equals(5) ? loc.frm_location_address.firstordefault(c => !c.removed && c.revision == 0).street1 : loc.name,             locationlevel = loc.frm_location_level.systemid,             participation = loc.isactive ? "yes" : "no",         });      var meters = db.frm_connection_meter         .where(x => !x.removed && x.revision == 0)         .select(l => new treedata         {             locationlevel = 6,             id = l.id,             parentlocationid = l.frm_location.id,             name = l.meternumber,             participation = l.ismain ? "yes" : "no",// change isactive after db update         });      return locations.concat(meters).tolist(); }