Get items from a list by ids of another list

Question

Sorry if question title is confusing, I have a Scenario where i have two class User and the Address.

public class User
{
    public int userId { get; set; }
    public string userName { get; set; }

    public List<Address> address { get; set; }
}

public class Address
{
    public string AddressLine { get; set; }
    public string City { get; set; }
    public string State { get; set; }

    public int UserId { get; set; }
}

UserId is foreign key in class address, I have two List, List of user and List of address. Values for address in User is null, I want to get list of address for each user how can i achieve that? I know we can use foreach, but is there way to achieve this by linq? Below is sample code.

class Program
{
    static void Main(string[] args)
    {
        var users = new List<User>();
        users.Add(new User { userId = 1, userName = "User 1" });
        users.Add(new User { userId = 2, userName = "User 2" });
        users.Add(new User { userId = 3, userName = "User 3" });
        users.Add(new User { userId = 4, userName = "User 4" });

        var address = new List<Address>();
        address.Add(new Address { AddressLine = "Address 1", City = "City 1", State = "State 1", UserId = 1 });
        address.Add(new Address { AddressLine = "Address 1 Dup", City = "City 1", State = "State 1", UserId = 1 });
        address.Add(new Address { AddressLine = "Address 2", City = "City 2", State = "State 2", UserId = 2 });
        address.Add(new Address { AddressLine = "Address 2 Dup", City = "City 2", State = "State 2", UserId = 2 });
        address.Add(new Address { AddressLine = "Address 3", City = "City 3", State = "State 3", UserId = 3 });
        address.Add(new Address { AddressLine = "Address 3 Dup", City = "City 3", State = "State 3", UserId = 3 });


    }
}

Answer 1

Linq version

List<User> result = users.Select(x => 
                         new User() { 
                            userId = x.userId, 
                            userName = x.userName, 
                            address = address.Where(y => y.UserId == x.userId).ToList() 
                         }).ToList();

---

imo more readable foreach version

foreach (User user in users)
    user.address = address.Where(y => y.UserId == user.userId).ToList();

Answer 2

It's rather simple with a Select and Where .

users = users.Select(u => { u.address = address.Where(a => a.UserId == u.userId).ToList(); return u; }).ToList();

A more readable version:

    users = users.Select(u =>
    {
        u.address = address.Where(a => a.UserId == u.userId).ToList();

        return u;
    }).ToList();