c# - Left join Lists with Linq and detect multiple matches -

my goal compare lists , update values on list values list b. along that, want work left join , keep values list if didn't updated , detect multiple matches.

what have tried far below.

the setup

public class person {     public string firstname { get; set; }     public string lastname { get; set; }     public string updateme { get; set; }      public static list<person> createlist()     {         return new list<person>         {             new person { lastname = "barton", firstname = "clint" },             new person { lastname = "stark", firstname = "tony" },             new person { lastname = "parker", firstname = "peter" }            };     }      public static list<person> createlisttwo()     {         return new list<person>         {             new person { lastname = "barton", firstname = "clint", updateme = "updated"},             new person { lastname = "stark", firstname = "tony", updateme = "updated"},         };     } }  public class firstlastnamematch : iequalitycomparer<person> {     public bool equals(person x, person y)     {         return x.firstname == y.firstname              && x.lastname == y.lastname;     }      public int gethashcode(person obj)     {         unchecked         {             var hash = 17;             hash = hash * 23 + obj.firstname.gethashcode();             hash = hash * 23 + obj.lastname.gethashcode();             return hash;         }     } } 

implementation

static void main() {     var lista = person.createlist();     var listb = person.createlisttwo();      //attempt     var result = lista.join(         listb,         x => x,         y => y,         (x, y) => new person         {             firstname = x.firstname,             lastname = x.lastname,             updateme = y.updateme         },         new firstlastnamematch()         );      foreach (var person in result)     {         console.writeline($"name: {person.firstname} {person.lastname} " +                           $"updateme: {person.updateme} ");     } } 

there problem run here is inner join , not left join. have found way left join cannot figure out how pass in iequalitycomparer syntax.

var result = persona in lista              join personb in listb on new {persona.firstname, persona.lastname } equals new { personb.firstname, personb.lastname }                  buffer              subperson in buffer.defaultifempty()              select new person              {                  firstname = persona.firstname,                  lastname = persona.lastname,                  updateme = (subperson == null ? string.empty : subperson.updateme)              }; 

the real catch once able complete left join while injecting comparison criteria, still need detect duplicates. need able identify if person lista matches more 1 person in listb. alright if lista had duplicates though.

end goal

left join 2 lists dynamic matching criteria.
detect multiple matches right list.
need update static list of properties on left list match right list.