How Do I Filter Out The Common Elements Of Two Lists?

If you want the values which are in both lists:

var inBoth = firstList.Intersect(secondList);

If you want the values which are in the first list, but not the second:

var inFirst = firstList.Except(secondList);

If you want the values which are not common to both lists:

var notInBoth = firstList.Except(secondList).Union(secondList.Except(firstList));