using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var  List1  =  new  List<Person>{

            new  Person{  

                id  =  1,

                fnName  = "bla"

            },  new  Person{  

                id  =  2,

                fnName  = "bla2"

            }

        };

        var  List2  =  new  List<Payment>{

            new  Payment{  

                paymentId  =  1,

                paymentType  = "type1"

            },  new  Payment{  

                paymentId  =  2,

                paymentType  = "type2"

            }

        };

        List2.ForEach(l2 => {

            l2.paymentPersonIds.ToList().ForEach(id  => {

                var matched =  List1.FirstOrDefault(l1 => l1.id.ToString().Contains(id));

                if(matched !=  null){

                    l2.PersonDetails.Append(matched);

                }

            });

        });

    }

}

​

public class Person

{

   public  int id;

   public  string fnName;

   public  string lnName;

}

​

public class Payment

{

   public  int paymentId;

   public  string paymentType;

   public  IEnumerable<string> paymentPersonIds;

   public  IEnumerable<Person> PersonDetails;

}