LINQ - Find all items in one list that aren't in another list

Try using .Except extension method ^(docs):

var result = list1.Except(list2);

will give you all items in list1 that are not in list2.

IMPORTANT: Even though there's a link provided to MSDN docs for the method, I'll point this out here: Except only works out of the box for collections of primitive types, for POCOs/objects you need to implement IEquatable on that object.

Try this:

var List2 = OriginalList.Where(item => !List1.Any(item2 => item2.ID == item.ID));

The easiest way is to use the Except method.

var deletedItems = list1.Except(joinItems);

This will return the set of items in list1 that's not contained in joinItems