c# - Skip all items after first one with same id -

assume have list<customobject> list.

customobject has next structure :

public class customobject {   public int coid{get;set;}   public list<subcustobj> sublist{get;set;}   public datetime datepublish{get;set;} } public class subcustobj {   public int id{get;set;}   public region rregion{get;set;}  } // region : it's important know has regionid 

if in list exists customobject's same coid in region need leave 1 element recent datepublish. in other words, 1 coid - 1 region. how can it?

//list it's list<customobject>     var todelete = ist.selectmany(r =>                                        r.sublist.where(m => r.sublist.count() > 2)                                         .select(rm => rm.rregion.regionid)); 

this code select ids of region repeated, todo next don't know.

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some new idea : can group elements regionid , after select first element in group?

you can select pairs (region, customobject), order datepublish, group region, , pick first item each group:

var res = list     .selectmany(c => c.sublist.select(s => new {custom = c, region = s.rregion}))     .orderbydescending(p => p.custom.datepublish)     .groupby(p => p.regionid)     .select(g => g.first()); 

this gives pairs of customobject, region such regions not repeated (customobjects may repeated, though).