Except has similar effect to Distinct?

Question

I just discovered that Except() will remove all elements in the second list from the first, but it also has the effect that it makes all elements in the returne

Answer 1

You wrote:

Simple way around I am using is Where(v => !secondList.Contains(v))

When you do this, there is still Distict done with secondList.

For example:

var firstStrings = new [] { "1", null, null, null, "3", "3" };
var secondStrings = new [] { "1", "1", "1", null, null, "4" };
var resultStrings = firstStrings.Where(v => !secondStrings.Contains(v)); // 3, 3  

I created an extension method to have no distinct at all. Examle of usage:

var result2Strings = firstStrings.ExceptAll(secondStrings).ToList(); // null, 3, 3

This is what it does:

This is the source:

public static IEnumerable ExceptAll(
    this IEnumerable first,
    IEnumerable second)
{
    // Do not call reuse the overload method because that is a slower imlementation
    if (first == null) { throw new ArgumentNullException("first"); }
    if (second == null) { throw new ArgumentNullException("second"); }

    var secondList = second.ToList();
    return first.Where(s => !secondList.Remove(s));
}

public static IEnumerable ExceptAll(
    this IEnumerable first,
    IEnumerable second,
    IEqualityComparer comparer)
{
    if (first == null) { throw new ArgumentNullException("first"); }
    if (second == null) { throw new ArgumentNullException("second"); }
    var comparerUsed = comparer ?? EqualityComparer.Default;

    var secondList = second.ToList();
    foreach (var item in first)
    {
        if (secondList.Contains(item, comparerUsed))
        {
            secondList.Remove(item);
        }
        else
        {
            yield return item;
        }
    }
}

Edit: A faster implemetation, based on the comment of DigEmAll

public static IEnumerable ExceptAll(
        this IEnumerable first,
        IEnumerable second)
{
    return ExceptAll(first, second, null);
}

public static IEnumerable ExceptAll(
    this IEnumerable first,
    IEnumerable second,
    IEqualityComparer comparer)
{
    if (first == null) { throw new ArgumentNullException("first"); }
    if (second == null) { throw new ArgumentNullException("second"); }


    var secondCounts = new Dictionary(comparer ?? EqualityComparer.Default);
    int count;
    int nullCount = 0;

    // Count the values from second
    foreach (var item in second)
    {
        if (item == null)
        {
            nullCount++;
        }
        else
        {
            if (secondCounts.TryGetValue(item, out count))
            {
                secondCounts[item] = count + 1;
            }
            else
            {
                secondCounts.Add(item, 1);
            } 
        }
    }

    // Yield the values from first
    foreach (var item in first)
    {
        if (item == null)
        {
            nullCount--;
            if (nullCount < 0)
            {
                yield return item;
            } 
        }
        else
        {
            if (secondCounts.TryGetValue(item, out count))
            {
                if (count == 0)
                {
                    secondCounts.Remove(item);
                    yield return item;
                }
                else
                {
                    secondCounts[item] = count - 1;
                }
            }
            else
            {
                yield return item;
            }
        }
    }
}

More info on my blog (also variant for Intersect and Union)