How to use Linq in 2 lists so that one complements the other in C#

Question

Hello, I have the following situation:

public class Foo {
  int id { get; set; }
  Bar bar { get; set; }
}
public class Bar {
  int id { get; set; }
  int FooId { get; set; }
}
public class Program {
  public static void Main(string[] args){
    var foos = new List<Foo> {
      new Foo{ id: 1 }, new Foo{ id: 2 }, 
      new Foo{ id: 3 }, new Foo{ id: 4 }
    };
    var bars = new List<Bar> {
      new Bar{ id: 5, FooId: 2},
      new Bar{ id: 6, FooId: 4}
    }
  }
}

So I can insert the bars within their respective foos i have to use a foreach to check if the items have property FooId same as Foo id on the list, so:

foreach(var foo in foos)
  foreach(var bar in bars)
    if(bar.FooId == foo.Id)
      foo.Bar = bar;

If I were to do it in Linq, I would have to generate a new object and assign all the properties of the old one to the new one and then correlate, like this:

foos = foos
  .Join(bars,foo => foo.Id,bar => bar.FooId, (foo, bar) => new { foo, bar })
  .Select(x=> new Foo{ Id = x.foo.Id, Bar = x.bar })
  .ToList();

Is there any way I can harness the object of foos without the need to recreate it? The idea of this question is because I can do something similar in javascript like this:

var foos = [{id: 1}, {id: 2}, {id: 3}, {id: 4}];
var bars = [{id: 5, fooId: 2}, {id: 6, fooId: 4}];

foos = foos.map(x=> x.bar = bars.filter(y => y.fooId === x.id)[0])

Answer 1

Your two examples (in c#) do not present the same results! The first (with chained foreach) the list gets the 4 elements and the second (with Join and select) the list gets only the 2 elements where the bar is not null

You can get the same result like this:

foreach (var foo in foos)
    foo.bar = bars.FirstOrDefault(b => b.FooId == foo.id);

It would actually be LastOrDefault..

Or so:

foos.ForEach(f => f.bar = bars.FirstOrDefault(b => b.FooId == f.id));

Or:

foos = foos.Select(f => { f.bar = bars.FirstOrDefault(b => b.FooId == f.id); return f; }).ToList();

To remove the nulls:

foos.RemoveAll(f => f.bar == null);