c# - Using Linq to group by multiple columns in a list and mark all duplicate items -

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using linq i'd query list , find duplicate persons (a duplicate defined having same first, last name , date of birth) , mark each of duplicate person's stateofdata property string "duplicate" , each unique person's stateofdata property string "unique".

public  class person  { public string personfirstname { get; set; } public string personlastname { get; set; } public datetime persondateofbirth { get; set; } public string stateofdata{ get; set }  public person (string firstname, string lastname, datetime dateofbirth,string state) {     this.personfirstname  = firstname;     this.personlastname = lastname;     this.persondateofbirth = dateofbirth;     this.stateofdata = state; }   }   ilist<person> personslist  =  new list<person>();  person pers = new person("diane","jones","1967-01-01",""); personslist.add(pers); person pers = new person("diane","jones","1967-01-01",""); personslist.add(pers); person pers = new person("john","jones","1967-01-01",""); personslist.add(pers);

the result of persons list should read:

"diane","jones","1967-01-01","duplicate"
"diane","jones","1967-01-01","duplicate"
"john","jones","1967-01-01","unique"

any appreciated

var thelookup = personlist   .groupby(p => new {p.personfirstname, p.personlastname, p.persondateofbirth})   .tolookup(g => g.skip(1).any() ? "duplicate" : "unique");  foreach(var lookupentry in thelookup) {   string stateofdata = lookupentry.key;   foreach(person p in lookupentry.selectmany(g => g))   {     p.stateofdata = stateofdata;   } }

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