how to create this linq query

Question

I have a situation where I have a list of ids such as {1,2,3,4}. I need to pass into a method another list of ids and if the list has the same numbers in it return true, else if either list is not identical (disregarding ordering) I need to return false. So, a call to the method with {1,2,3,4,5} should return false while a call with {2,4,1,3} returns true. This sounds simple enough but I can't figure out how to do it.

Answer 1

The simplest way is probably this:

var idSet = new HashSet<int>(idList1);
if (idSet.SetEquals(idList2))
{
    ...
}

As per the comments, this will consider {1, 1, 1, 1, 1} to be equal to {1} - in other words, it treats it as a set instead of an unordered collection of possibly-duplicate values.

Answer 2

If you are guaranteed to not have repeating elements in idList, you can use the following:

if (idList.Count == otherIDList.Count &&
    idList.Intersect(otherIDList).Count() == idList.Count)
{
    // Contain same things.
}
else
{
    // Do not contain same things.
}

The first check is to make sure they're the same size. This is a really cheap way to see if the lists even have a chance of being identical, same as with strings. Also, without it, the statement will return true if otherIDList is a superset of idList.

If you cannot guarantee uniqueness within the collection, I think you're going to have to code something yourself.