how does except method work in linq

Question

I have the classes:

class SomeClass
{
   public string Name{get;set;}
   public int SomeInt{get;set;}
}


class SomeComparison: IEqualityComparer

        

Answer 1

It seems to me this would be more efficient

private static IEnumerable ExceptImpl(
    IEnumerable first,
    IEnumerable second,
    IEqualityComparer comparer)
{
    HashSet bannedElements = new HashSet(second, comparer);
    foreach (TSource item in first)
    {
        if (!bannedElements.Contains(item))
        {
            yield return item;
        }
    }
}

Contains is O(1)

Add is if Count is less than the capacity of the internal array, this method is an O(1) operation. If the HashSet object must be resized, this method becomes an O(n) operation, where n is Count.