c# - Can I check the IEnumerable<T> before using yield return -

i have creating ienumerable stream creating each record 1 one. there way check current results if id exists don't want add read record set.

public class employee {     public string name { get; set; }     public string country { get; set; }     public string rank { get; set; }     public string hours { get; set; }      public static employee create(idatarecord record)     {         return new employee         {            name = record["name"],            country = record["country"],            rank = record["rank"],            hours = record["hours"],         };     } }  public ienumerable<employee> getemployees(textreader stream) {     using (var reader = mylibraryfunction(stream)     {        while (reader.read())        {            yield return employee.create(reader);        }     } }  var result = getemployees(stream); 

edit: simplified example. want know if can access existing records in set. if exists, update 1 field (add number of hours) in employee, if not exist should added.

use hashset of yielded object id`s :

public ienumerable<employee> getemployees(textreader stream) {     var exisiting = new hashset<int>();     using (var reader = mylibraryfunction(stream)     {         while (reader.read())         {             var employee = employee.create(reader);             if (exisiting.add(employee.id)) {                 yield return employee;             }         }     } }