List of Objects with int property compared to List of Int

Question

I have 2 lists. First is a list of objects that has an int property ID. The other is a list of ints.

I need to compare these 2 lists and copy the objects to a new list with only the objects that matches between the two lists based on ID. Right now I am using 2 foreach loops as follows:

var matched = new list<Cars>();
foreach(var car in cars)
foreach(var i in intList)
{
 if (car.id == i) 
  matched.Add(car);
}

This seems like it is going to be very slow as it is iterating over each list many times. Is there way to do this without using 2 foreach loops like this?

Answer 1

One slow but clear way would be

var matched = cars.Where(car => intList.Contains(car.id)).ToList();

You can make this quicker by turning the intList into a dictionary and using ContainsKey instead.

var intLookup = intList.ToDictionary(k => k);
var matched = cars.Where(car => intLookup.ContainsKey(car.id)).ToList();

Even better still, a HashSet:

var intHash = new HashSet(intList);
var matched = cars.Where(car => intHash.Contains(car.id)).ToList();

Answer 2

You could try some simple linq something like this should work:

var matched = cars.Where(w => intList.Contains(w.id)).ToList(); 

this will take your list of cars and then find only those items where the id is contained in your intList.