Question

why linq’s except extension method does not have Except<TSource> Method (IEnumerable<TSource>,HashSet<TSource>) overload?

for example

  var query = A.Except(B).where(x=>Criteria(x))

  foreach(item in query)
   {
     B.add(item);
     DoSomething(item);
   }

Given B is HashSet<T>, A is IEnumerable<T> or ICollection<T>
Here in each iteration Except take O(|B|) time.
why there is not a method that just take O(1) time, as B is Hashset anyway.

Update

my crude way is

var query = A.where(x=>!B.contains(x)).where(x=>Criteria(x))

Answer 1

Here in each iteration Except take O(|B|) time

No it doesn’t. Except is implemented internally with a collection similar to HashSet<T>. You can have a look at Jon Skeet’s proposed implementation in Edulinq. All elements in B are placed in a HashSet<T>, then elements of A are enumerated; if they’re not in the HashSet (checking this is a O(1) operation), they are returned in the output sequence.