The opposite of Intersect()

Question

Intersect can be used to find matches between two collections, like so:

// Assign two arrays.
int[] array1 = { 1, 2, 3 };
int[] array2 = { 2, 3, 4 };
// Call Intersect extension method.
var intersect = array1.Intersect(array2);
// Write intersection to screen.
foreach (int value in intersect)
{
    Console.WriteLine(value); // Output: 2, 3
}

However what I'd like to achieve is the opposite, I'd like to list the items that are missing when comparing two collections:

// Assign two arrays.
int[] array1 = { 1, 2, 3 };
int[] array2 = { 2, 3, 4 };
// Call Intersect extension method.
var intersect = array1.NonIntersect(array2); // I've made up the NonIntersect method
// Write intersection to screen.
foreach (int value in intersect)
{
    Console.WriteLine(value); // Output: 1, 4
}

Answer 1

/// <summary>
/// Given two list, compare and extract differences
/// http://stackoverflow.com/questions/5620266/the-opposite-of-intersect
/// </summary>
public class CompareList
{
    /// <summary>
    /// Returns list of items that are in initial but not in final list.
    /// </summary>
    /// <param name="listA"></param>
    /// <param name="listB"></param>
    /// <returns></returns>
    public static IEnumerable<string> NonIntersect(
        List<string> initial, List<string> final)
    {
        //subtracts the content of initial from final
        //assumes that final.length < initial.length
        return initial.Except(final);
    }

    /// <summary>
    /// Returns the symmetric difference between the two list.
    /// http://en.wikipedia.org/wiki/Symmetric_difference
    /// </summary>
    /// <param name="initial"></param>
    /// <param name="final"></param>
    /// <returns></returns>
    public static IEnumerable<string> SymmetricDifference(
        List<string> initial, List<string> final)
    {
        IEnumerable<string> setA = NonIntersect(final, initial);
        IEnumerable<string> setB = NonIntersect(initial, final);
        // sum and return the two set.
        return setA.Concat(setB);
    }
}

Answer 2

I'm not 100% sure what your NonIntersect method is supposed to do (regarding set theory) - is it
B \ A (everything from B that does not occur in A)?
If yes, then you should be able to use the Except operation (B.Except(A)).