C# compare two Lists and update it's column values -

i've got 2 c# lists (list lista , list listb) how can compare these 2 , if duplicate (of specific columns ex. id_num , id_cust) found update column "id_duplicate" value of listb's columns id.

dataset ds =  submain;  list<string> lista = (from r in ds.tables[0].asenumerable()                       select r.field<string>("id_num") +                         r.field<string>("id_cust")).tolist();  dataset dsmain = mains;  list<string> listb = (from r in dsmain.tables[0].asenumerable()                       select r.field<string>("id_num") +                        r.field<string>("id_cust")).tolist(); 

i want lista contain new column id_duplicate value id_num listb.

so duplicates somehow linked id_num.

i update id_duplicate database.

edit: added more explanation in comment bellow.

if understand join:

var lista = new list<row> {      new row { id= 1, idnum = 1, idcust = 1 },      new row { id= 2, idnum = 1, idcust = 2 },      new row { id= 3, idnum = 2, idcust = 1 },      new row { id= 4, idnum = 1, idcust = 3 },      new row { id= 5, idnum = 3, idcust = 1 },      new row { id= 6, idnum = 4, idcust = 1 }  };  var listb = new list<row> {      new row { id= 1, idnum = 5, idcust = 1 },      new row { id= 5, idnum = 6, idcust = 2 },      new row { id= 7, idnum = 2, idcust = 1 },      new row { id= 9, idnum = 1, idcust = 3 },      new row { id= 11, idnum = 7, idcust = 2 } };      var t = (from in lista              join b in listb on a.idcust.tostring() + a.idnum.tostring()               equals               b.idcust.tostring() + b.idnum.tostring()              select new              {                 id = a.id,                 idupdate = b.id              }).toarray();  foreach (var item in t) {     console.writeline("id {0} idupdate {1}", item.id, item.idupdate); } 

here row class

class row {     public int id { get; set; }      public int idnum { get; set; }      public int idcust { get; set; } } 

obviusly can create calculated column on row class this

public string valuetocompare {       {       return this.idnum.tostring() + this.idcust.tostring();    } } 

and use on join comparison

max