Linq find differences in two lists

Question

I have two list of members like this:

Before: Peter, Ken, Julia, Tom

After: Peter, Robert, Julia, Tom

As you can see, Ken is is out and Robert is in.

Answer 1

Here is the version having O(n) complexity, provided your sequences are both ordered:

    public static IEnumerable SymmetricDifference(IEnumerable coll1, IEnumerable coll2, IComparer cmp)
    {
        using (IEnumerator enum1 = coll1.GetEnumerator())
        using (IEnumerator enum2 = coll2.GetEnumerator())
        {
            bool enum1valid = enum1.MoveNext();
            bool enum2valid = enum2.MoveNext();
            while (enum1valid && enum2valid)
            {
                int cmpResult = cmp.Compare(enum1.Current, enum2.Current);
                if (cmpResult < 0)
                {
                    yield return enum1.Current;
                    enum1valid = enum1.MoveNext();
                }
                else if (cmpResult > 0)
                {
                    yield return enum2.Current;
                    enum2valid = enum2.MoveNext();
                }
                else
                {
                    enum1valid = enum1.MoveNext();
                    enum2valid = enum2.MoveNext();
                }
            }
            while (enum1valid)
            {
                yield return enum1.Current;
                enum1valid = enum1.MoveNext();
            }
            while (enum2valid)
            {
                yield return enum2.Current;
                enum2valid = enum2.MoveNext();
            }
        }
    }


    public static IEnumerable SymmetricDifference(IEnumerable coll1, IEnumerable coll2)
    {
        return SymmetricDifference(coll1, coll2, Comparer.Default);
    }