Оптимизация кода, метод Contains

public class A
{
public int Id {get;set;}
public string Description {get;set;}
public A (int id, string description)
{
Id = id;
Description = description;
}
}

List<A> listA = new List<A>();
listA.Add(1, "A");
listA.Add(2, "B");
listA.Add(3, "С");

List<int> IDlist = new List<int>();
for (int i=0; i<listA.count; i++)
{
IDlist.Add(listA[i].Id);
}
 
if (IDlist.Contains(2)) {}

List<A> listA = new List<A>();
listA.Add(1, "A");

listA.Add(new A(1, "A"));

            List<A> listA = new List<A>();
            int id = 2;
            var a = listA.FirstOrDefault(obj => obj.Id == id);

for (int i=0; i<listA.count; i++)
if (listA[i].Id == 2)
{
    //....
    break;
};

public B
{
  public A A {get;set;}
}

List<int> IDlist = new List<int>();
for (int i=0; i<listA.count; i++)
{
IDlist.Add(listA[i].Id);
}
 
listB = listB.Where(x => IDlist.Contains(x.A.Id)).ToList();

foreach (var item in listA)
if (item.Id == 2) Console.WriteLine(item.Id + "|" + item.Description);

listB = listB.Where(x => listA.Any(a => a.Id == x.A.id)).ToList();

            ICollection<int> condition = listA.Select(x=>x.Id).ToHashSet();
            List<A> matched = listB.Where(x=>condition.Contains(x.Id)).ToList();

   List<A> matched = listB.Intersect(listA, (x,y)=>x.Id==y.Id).ToList();

ICollection<int> condition = listA.Select(x=>x.Id).ToHashSet();
* * * * * * List<B> matched = listB.Where(x=>condition.Contains(x.A.Id)).ToList();