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
					
public class Program
{
	public static int loopcnt = 0;
	
	public static void Main()
	{
		    Stopwatch sw = new Stopwatch();
            sw.Start(); 
            
		    //RANDOM case - typical use ~ 78 ticks 
		    string[] arrA = new string[] { "7", "1", "2", "3", "4", "5", "9", "7", "7", "1" };
            string[] arrB = new string[] { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };

		    //WANT           2,5,5,5,9,7,7,7
		
            //WORST case - entirely different, but have 1 value in common to make it interesting ~ 88 ticks
		    //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 ~ 24 ticks
		    //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));
            sw.Restart();
            
        	string[] diffAminusB = new string[arrA.Length];      
			string[] diffBminusA = new string[arrB.Length];
             
            BitArray bitarrRemoveA = new BitArray(arrA.Length); 
                        
            int d = 0; //index for diff list
            int a = 0; 
            bool nf = true; //not found 

			//bitarrRemove.Set(a, true);  version -> 13 ticks blazing fast, 19 ticks for worst case identical sets on my computer!
            for (int b = 0; b < arrB.Length; b++)
            {
                nf = true;

                for (a = 0; a < arrA.Length; a++)
                {
					loopcnt++; //not require for metrics
					//diffAminusB[a] = arrA[a]; //Tried to preload this array, and remove items using speedy BitArray, but slow				

                    if (!bitarrRemoveA[a] && arrA[a] == arrB[b])
                    {
                        nf = false;
                        bitarrRemoveA.Set(a, true); //magic - A-B = diffsA - flag removals for post processing
                        break;

                    }
                }

                if (nf) //not found in set B 
                    diffBminusA[d++] = arrB[b]; //diffs B not found in A, mathematically B-A = diffsB (order important here, driven by B, so those results appear)
                    
                
				
			}
			
		    //Get A-B = diffsA, some memory savings (but very slight) here since it's a sized array
            int idx = 0;
            for (a = 0; a < arrA.Length; a++)
            {
                if (!bitarrRemoveA[a])
                    diffAminusB[idx++] = arrA[a];
               
            }

	
	   	   sw.Stop();

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

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

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

		   //Un	
           Console.WriteLine("Result Unorderd Final A then B = " + (string.Join(",", diffAminusB)).Trim(',') + "," + string.Join(",", diffBminusA).Trim(',') + " in " + sw.ElapsedTicks + " ticks.");
           Console.WriteLine("Result Unorderd Final B then A = " + (string.Join(",",  diffBminusA)).Trim(',') + "," + string.Join(",",diffAminusB).Trim(',') + " in " + sw.ElapsedTicks + " ticks.");

            
           List<string> C = diffBminusA.ToList();

           List<string> D = diffBminusA.ToList().Concat(diffAminusB.ToList()).ToList();
           D.Sort();

           Console.WriteLine("Result  Ordered Final = " + string.Join(",", D).TrimStart(','));
		   Console.WriteLine("Result  O(n^2) but very fast - can you beat it?"); 
	
	}
}

           Console.WriteLine("Result Unorderd Final A then B = " + (string.Join(",", diffAminusB)).Trim(',') + "," + string.Join(",", diffBminusA).Trim(',') + " in " + sw.ElapsedTicks + " ticks.");
 
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
                    
public class Program
{
    public static int loopcnt = 0;
    
    public static void Main()
    {
            Stopwatch sw = new Stopwatch();
            sw.Start(); 
            
            //RANDOM case - typical use ~ 78 ticks 
            string[] arrA = new string[] { "7", "1", "2", "3", "4", "5", "9", "7", "7", "1" };
            string[] arrB = new string[] { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };
​
            //WANT           2,5,5,5,9,7,7,7