Performance between check exists before add to list and distinct in linq

Question

In the foreach loop, I want to add the Products to a List, but I want this List to not contain duplicate Products, currently I have two ideas solved.

1/ In the loop, before adding the Product to the List, I will check whether the Product already exists in the List, otherwise I will add it to the List.

foreach (var product in products)
{
    // code logic
    if(!listProduct.Any(x => x.Id == product.Id))
    {
        listProduct.Add(product);
    }
}

2/. In the loop, I will add all the Products to the List even if there are duplicate products. Then outside of the loop, I would use Distinct to remove duplicate records.

foreach (var product in products)
{
    // code logic
        listProduct.Add(product);
}
listProduct  = listProduct.Distinct().ToList();

I wonder in these two ways is the most effective way. Or have any other ideas to be able to add records to the List to avoid duplication ??

Answer 1

I'd go for a third approach: the HashSet. It has a constructor overload that accepts an IEnumerable. This constructor removes duplicates:

If the input collection contains duplicates, the set will contain one of each unique element. No exception will be thrown.

Source: HashSet<T> Constructor

usage:

List<Product> myProducts = ...;
var setOfProducts = new HashSet<Product>(myProducts);

After removing duplicates there is no proper meaning of setOfProducts[4].

Therefore a HashSet is not a IList<Product>, but an ICollection<Product>, you can Count / Add / Remove, etc, everything you can do with a List. The only thing you can't do is fetch by index