Linq Except Indistinct

ExceptIndistinctExtensions.cs

using System;
using System.Linq;
using System.Collections.Generic;
                    
public class Program
{
    public static void Main()
    {
        // Credit to Alex Siepman and digEmAll on StackOverflow (answer https://stackoverflow.com/a/18321594/1049710)
        
        // Demonstration
        var newValues = new string[] { "A",      "C", "D", "D", "E",      "F", "F", "G", "G" };
        var oldValues = new string[] { "A", "B", "C",           "E", "E",           "G"      };

        Console.WriteLine("Except             in new but not in old:   " + String.Join(",", newValues.Except(oldValues)));
        // output: D,F
        Console.WriteLine("Except             in old but not in new:   " + String.Join(",", oldValues.Except(newValues)));
        // output: B
        Console.WriteLine("ExceptIndistinct   in new but not in old:   " + String.Join(",", newValues.ExceptIndistinct(oldValues)));
        // output: D,D,F,F,G
        Console.WriteLine("ExceptIndistinct   in old but not in new:   " + String.Join(",", oldValues.ExceptIndistinct(newValues)));
        // output: B,E
    }
}

public static class ExceptIndistinctExtensions
{
    /// <summary>Returns a list of all values except those also found in given second list.</summary>
    public static IEnumerable<TSource> ExceptIndistinct<TSource>(
        this IEnumerable<TSource> first,
        IEnumerable<TSource> second)
    {
        return ExceptIndistinct(first, second, null);
    }

    /// <summary>Returns a list of all values except those also found in given second list.</summary>
    public static IEnumerable<TSource> ExceptIndistinct<TSource>(
        this IEnumerable<TSource> first,
        IEnumerable<TSource> second,
        IEqualityComparer<TSource> comparer)
    {
        if (first == null) { throw new ArgumentNullException("first"); }
        if (second == null) { throw new ArgumentNullException("second"); }


        var secondCounts = new Dictionary<TSource, int>(comparer ?? EqualityComparer<TSource>.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;
                }
            }
        }
    }
}