C# Online Compiler | .NET Fiddle

using System;
using System.Collections; 
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics; 

//CHALLENGE - Get difference between two unorderd, not unique lists, duplicates allowed within a list and across both lists.
//SOLUTION - https://metadataconsulting.blogspot.com/2021/02/CSharp-dotNet-Get-difference-between-two-unordered-not-unique-lists-aka-duplicates-allowed-within-a-list-and-across-both-lists.html
//OPTIMIZE - THIS IS IT using BitArray - https://metadataconsulting.blogspot.com/2021/02/CSharp-dotNet-Get-difference-between-two-unordered-not-unique-lists-lightning-fast-optimized.html
//OPTIMIZEv2 - All BitArray imp - https://metadataconsulting.blogspot.com/2021/02/CSharp-dotNet-Get-difference-between-two-unordered-not-unique-lists-using-BitArrays-for-super-fast-speed.html
//           - use ExceptIndistinct for >100 items 
public static class Program
{
	public static int loopcnt = 0;
	
	//https://stackoverflow.com/questions/2975944/except-has-similar-effect-to-distinct
	public static IEnumerable<T> ExceptFrom<T>(
		this IEnumerable<T> first,
		IEnumerable<T> second) //,
		//IEqualityComparer<TSource> comparer)
		{
			if (first == null)
				throw new ArgumentNullException();

			if (second == null)
				throw new ArgumentNullException();

			//var secondSet = second as HashSet<TSource> ?? // this trick ignore the comparer
			//				second.ToHashSet(comparer ?? EqualityComparer<TSource>.Default);

		    var secondSet = second as HashSet<T> ?? // this trick ignore the comparer
							second.ToHashSet(EqualityComparer<T>.Default);

			// Contains is O(1) for HashSet.
			return first.Where(v => !secondSet.Equals(v)); //change to equals, not Contains
		}

	//use after 100 items
    public static IEnumerable<T> ExceptIndistinct<T>(this IEnumerable<T> first, IEnumerable<T> second) 
	{
		if(first == null) { throw new ArgumentNullException("first"); }
		if(second == null) { throw new ArgumentNullException("second"); }

		var secondCounts = new Dictionary<T, int>(EqualityComparer<T>.Default);
		int count;
		int nullCount = 0;

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

		foreach(var item in first)
		{
			loopcnt++; 
			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; }
		}
	}

	public static void Main()
	{
			Stopwatch sw = new Stopwatch();
			

			//RANDOM case - typical use
			string[] arrA = new string[] { "a", "7", "1", "2", "3", "4", "5", "9", "7", "7", "1","222222222222222222","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222323" };
			string[] arrB = new string[] { "a", "1", "1", "2", "2", "3", "4", "5", "5", "5", "5","999999999999999999","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222323" };
			
			sw.Start(); 
			//WANT           2,5,5,5,9,7,7,7

			//WORST case - entirely different, but have 1 value in common to make it interesting
			//string[] arrA = new string[] { "1", "1", "2", "3", "4", "5", "6", "7", "7", "7" };//1st list
			//string[] arrB = new string[] { "8", "9", "10", "12", "1", "14", "15", "15", "11", "9" };//2nd list

			//BEST case - identical
			//string[] arrA = new string[] { "1", "1", "2", "3", "4", "5", "6", "7", "7", "7" };//1st list
			//string[] arrB = new string[] { "1", "1", "2", "3", "4", "5", "6", "7", "7", "7" };//2nd list


			sw.Stop();
			Console.WriteLine("RANDOM CASE -two unorderd, not unique lists ");
			Console.WriteLine("A  = " + string.Join(",", arrA));
			Console.WriteLine("B  = " + string.Join(",", arrB));
	    	Console.WriteLine("A count = " + arrA.Count());
			Console.WriteLine("B count = " + arrB.Count());
			
			sw.Restart();

			string[] arrBminusA = new string[arrB.Length];
			string[] arrAminuB = new string[arrA.Length];   
			BitArray bitArrAminusB = new BitArray(arrA.Length);
			BitArray bitarrBminusA = new BitArray(arrB.Length);
			
			//May seem counter intuitive to run this over 3 loop but it's faster ~ 10 ticks
			int a = 0;
            for (int b = 0; b < arrB.Length; b++)
            {
                for (a = 0; a < arrA.Length; a++)
                {
                    loopcnt++;//not need for timing analysis 

                    if (!bitArrAminusB[a] && arrA[a].Length == arrB[b].Length && arrA[a] == arrB[b])
                    {
                        bitArrAminusB.Set(a, true); //A-B, list A difference/subtract of list B  
                        bitarrBminusA.Set(b, true); //B-A, list B difference/subtract of list A  
                        break;

                    }
                }
            }

            int idxa = 0;
            for (int aa = 0; aa < arrA.Length; aa++)
            {
                if (!bitArrAminusB[aa])
                    arrAminuB[idxa++] = arrA[aa];
                
            }

            int idxb = 0;
            for (int bb = 0; bb < arrB.Length; bb++)
            {
                if (!bitarrBminusA[bb])
                    arrBminusA[idxb++] = arrB[bb];

            }
			sw.Stop();

           Console.WriteLine("(A-B) Result = " + string.Join(",", arrAminuB).Trim(','));
           //Console.WriteLine("D  = " + string.Join(",", D));
           //Console.WriteLine("E  = " + string.Join(",", E)); 

           Console.WriteLine("(B-A) Result = " + string.Join(",", arrBminusA).TrimEnd(','));

           Console.WriteLine("No. of loops = " + loopcnt);
           Console.WriteLine("No. of loops = " + arrA.Length*2 + " - Post copy using BitArray to a sized array");

		   //Un	
           Console.WriteLine("Result Unorderd Final A then B = " + (string.Join(",", arrAminuB)).Trim(',') + "," + string.Join(",", arrBminusA).Trim(',') + " in " + sw.ElapsedTicks + " ticks.");
           Console.WriteLine("Result Unorderd Final B then A = " + (string.Join(",",  arrBminusA)).Trim(',') + "," + string.Join(",",arrAminuB).Trim(',') + " in " + sw.ElapsedTicks + " ticks.");
 		   Console.WriteLine("Note: In a VS solution this is 47 ticks.");
           
           List<string> D = arrBminusA.ToList().Concat(arrAminuB.ToList()).ToList();
           D.Sort();

           Console.WriteLine("Result  Ordered Final = " + string.Join(",", D).TrimStart(','));
		   Console.WriteLine("Result  O(n^2+2n) but very fast - can you beat it?"); 
		
		   loopcnt = 0; 
		   Console.WriteLine("\n\nExceptIndistinct - Reddit Solution by UninformedPleb\n"); 
           sw.Reset(); 
		   
		   
		   List<string> listA = new List<string> { "a", "7", "1", "2", "3", "4", "5", "9", "7", "7", "1","222222222222222222","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222323" };
		   List<string> listB = new List<string>  { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };
		   
		   sw.Start(); 
           List<string> listC =  listA.ExceptIndistinct(listB).ToList();
		   List<string> listD =  listB.ExceptIndistinct(listA).ToList();

     	   sw.Stop();

		   Console.WriteLine("No. of loops = " + loopcnt);
		   Console.WriteLine("Result Unorderd Final A then B = " + (string.Join(",", listC)).Trim(',') + "," + string.Join(",", listD).Trim(',') + " \nin " + sw.ElapsedTicks + " ticks.");
			
		   loopcnt = 0; 
		   Console.WriteLine("\n\nExceptIndistinct - Reddit Solution by UninformedPleb using arrays\n"); 
           sw.Reset(); 
		   
		   string[] arrAA = new string[] { "a", "7", "1", "2", "3", "4", "5", "9", "7", "7", "1","222222222222222222","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222323" };
    	   string[] arrBB = new string[] { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5","999999999999999999","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222" };
  		   sw.Start(); 
		   
           string[] listCC =  arrAA.ExceptIndistinct(arrBB).ToArray(); 
		   string[] listDD =  arrBB.ExceptIndistinct(arrAA).ToArray(); 

     	   sw.Stop();
		
		   Console.WriteLine("No. of loops = " + loopcnt);
		   Console.WriteLine("Result Unorderd Final A then B = " + (string.Join(",", listC)).Trim(',') + "," + string.Join(",", listD).Trim(',') + " \nin " + sw.ElapsedTicks + " ticks.");
		
		
		
		   loopcnt = 0; 
		   Console.WriteLine("\n\nStackOverflow ExceptFrom\n"); 
           sw.Reset(); 
		 
		   string[] arrAAA = new string[] { "a", "7", "1", "2", "3", "4", "5", "9", "7", "7", "1","222222222222222222","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222323" };
    	   string[] arrBBB = new string[] { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5","999999999999999999","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222" };
  		   sw.Start(); 
		   
           string[] listCCC =  arrAAA.ExceptFrom(arrBBB).ToArray(); 
		   string[] listDDD =  arrBBB.ExceptFrom(arrAAA).ToArray(); 

     	   sw.Stop();
		
		   Console.WriteLine("No. of loops = " + loopcnt);
		   Console.WriteLine("Result Unorderd Final A then B = " + (string.Join(",", listC)).Trim(',') + "," + string.Join(",", listD).Trim(',') + " \nin " + sw.ElapsedTicks + " ticks.");
		
		
		
			
	}
}

           List<string> listA = new List<string> { "a", "7", "1", "2", "3", "4", "5", "9", "7", "7", "1","222222222222222222","22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222323" };
 
using System;
using System.Collections; 
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics; 
​
//CHALLENGE - Get difference between two unorderd, not unique lists, duplicates allowed within a list and across both lists.
//SOLUTION - https://metadataconsulting.blogspot.com/2021/02/CSharp-dotNet-Get-difference-between-two-unordered-not-unique-lists-aka-duplicates-allowed-within-a-list-and-across-both-lists.html
//OPTIMIZE - THIS IS IT using BitArray - https://metadataconsulting.blogspot.com/2021/02/CSharp-dotNet-Get-difference-between-two-unordered-not-unique-lists-lightning-fast-optimized.html
//OPTIMIZEv2 - All BitArray imp - https://metadataconsulting.blogspot.com/2021/02/CSharp-dotNet-Get-difference-between-two-unordered-not-unique-lists-using-BitArrays-for-super-fast-speed.html
//           - use ExceptIndistinct for >100 items 
public static class Program
{
    public static int loopcnt = 0;
    
    //https://stackoverflow.com/questions/2975944/except-has-similar-effect-to-distinct
    public static IEnumerable<T> ExceptFrom<T>(
        this IEnumerable<T> first,
        IEnumerable<T> second) //,
        //IEqualityComparer<TSource> comparer)
        {
            if (first == null)
                throw new ArgumentNullException();
​