Joining data from two API responses

I am getting data from two different API calls. One gets me "customer" data, the other "towns" data. Each customer has a town reference, so in my application, I need to have the actual town object referenced by the customer object. Here is a vastly simplified representation of my classes:

public class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int townId { get; set; }

    public Town town { get; set; }
}

public class Town
{
    public int townId { get; set; }
    public string townName { get; set; }
}

The only way I can think of achieving my objective is as follows (assume I am actually getting the lists from my API calls, the following is just for sake of explanation of the problem):

List<Customer> customers = new List<Customer>();

customers.Add(new Customer { Id = 1, Name = "First Customer", townId = 10 });
customers.Add(new Customer { Id = 2, Name = "Second Customer", townId = 20 });
customers.Add(new Customer { Id = 2, Name = "Third Customer", townId = 20 });

List<Town> towns = new List<Town>();

towns.Add(new Town { townId = 10, townName = "Eton" });
towns. Add(new Town { townId = 20, townName = "Harrow" });
towns.Add(new Town { townId = 30, townName = "Cambridge" });

foreach(Customer c in customers)
{
    c.town = towns.Where(t => t.townId == c.townId).FirstOrDefault();
}

This does not feel like an efficient way of achieving my objective, especially when I have dozens of other places I would need to do the same sort of thing.

>Solution :

As written according to your comment, you are using the original Customer object without assigning Town to the town property.

It should be:

For query expression

customers = (from a in customers
            join b in towns on a.townId equals b.townId
            select new Customer
            {
                Id = a.Id,
                Name = a.Name,
                townId = a.townId,
                town = b
            })
            .ToList();

For method expression

customers = customers
            .Join(towns,
                 x => x.townId,
                 x => x.townId,
                 (x, y) => 
                  {
                      var customer = x;
                      customer.town = y;
                      
                      return customer;
                  })
            .ToList();