How to subtract one huge list from another efficiently in C#

Question

I have a very long list of Ids (integers) that represents all the items that are currently in my database:

var idList = GetAllIds();

I also hav

Answer 1

Assuming the following premises are true:

• idList and itemsToAdd may not contain duplicate values
• you are using the .NET Framework 4.0

you could use a HashSet this way:

var itemsToAddSet = new HashSet(itemsToAdd);
itemsToAddSet.ExceptWith(idList);

According to the documentation the ISet.ExceptWith method is pretty efficient:

This method is an O(n) operation, where n is the number of elements in the other parameter.

In your case n is the number of items in idList.