C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics; 
					
public static class Program
{
	 public static int loopcnt = 0; 
	 public static IEnumerable<T> RemoveRange<T>(this IEnumerable<T> source, IEnumerable<T> second)
	 {
		 var tempList = source.ToList();

		 foreach (var item in second)
		 {
			 tempList.Remove(item); //this is cause a copy like RemoveAt below
		 }

		 return tempList;
	 }

	//RemoveAt for arrays, this is problematic copy and shrink time consuming - see my next post on how to fix this!
	public static void RemoveAt<T>(ref T[] arr, int index)
	{
		for (int a = index; a < arr.Length - 1; a++)
		{
			// moving elements downwards, to fill the gap at [index]
			arr[a] = arr[a + 1];
			loopcnt++; //not required but here for time anaylsis
		}
		// finally, let's decrement Array's size by one
		Array.Resize(ref arr, arr.Length - 1);
	}
    
	//CHALLENGE - Get difference between two unorderd, not unique lists, duplicates allowed within a list and across both lists.
	//Will demonstrate all solutions from https://stackoverflow.com/questions/15801655/difference-between-two-lists-preserving-duplicates 
	//are WRONG!
	//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 - Next blog post will drastically reduce increase the speed of this
	public static void Main()
	{
		
			Console.WriteLine("Get difference between two unordered, not unique (duplicates) lists!");
		    Console.WriteLine("Solution by 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");
			Console.WriteLine();
		
		 	Stopwatch sw = new Stopwatch();
            sw.Start(); 

            //UNORDERED, NOT UNIQUE lists, lists with duplicates 
            string[] arrA = new string[] { "1", "1", "2", "3", "4", "5", "9", "7", "7", "7" };
            string[] arrB = new string[] { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };

			//DESIRED RESULT = 2,5,5,5,7,7,7,9 OR 2,5,5,5,9,7,7,7 OR 9,7,7,7,2,5,5,5 UNORDERED IS SUFFICIENT, WE LOOKING FOR SPEED
		
            sw.Stop();
            Console.WriteLine("A  = " + string.Join(",", arrA));
            Console.WriteLine("B  = " + string.Join(",", arrB));
			Console.WriteLine("A Count  = " + arrA.Length);    
			Console.WriteLine("B Count  = " + arrB.Length);
		    Console.WriteLine();
			sw.Restart();

            //MY IMPLEMENTATION PURE ARRAY SOLUTION -     
		
            string[] diff = new string[arrA.Length+arrB.Length]; //contains B-A 
                        
            int d = 0; //index for diff list
            bool nf = true; //not found 
            loopcnt = 0; 
		
			int a = 0; 	
        	for (int b = 0; b < arrB.Length; b++)
            {
                nf = true;

                for (a = 0; a < arrA.Length; a++)
                {
                    if (arrA[a] == arrB[b])
                    {
                        nf = false;

                        RemoveAt(ref arrA, a); //magic - removes instance found, A is left with differences not found in B, mathematically A-B = diffsA 

                        break;

                    }
                }

                if (nf) //not found in set B 
                {
                    diff[d] = arrB[b]; //diffs B not found in A, mathematically B-A = diffsB (order important here, driven by B, so those results appear)
                    d++;
                }
				loopcnt++; //not required but here for time anaylsis
            }
            
            sw.Stop(); 
			Console.WriteLine("No. of loops = " + loopcnt);
            Console.WriteLine("(A-B) Result = " + string.Join(",", arrA).TrimEnd(','));
            Console.WriteLine("(B-A) Result = " + string.Join(",", diff).TrimEnd(','));
            Console.WriteLine("Result Unorderd Final = " + (string.Join(",", diff)).TrimEnd(',') + "," + string.Join(",", arrA) + " in " + sw.ElapsedTicks + " ticks.");

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

            List<string> D = diff.ToList().Concat(arrA.ToList()).ToList();
            D.Sort();

            Console.WriteLine("Result  Ordered Final = " + string.Join(",", D).TrimStart(','));
            
            sw.Reset();
            sw.Start();
			
		    Console.WriteLine(); 
            Console.WriteLine("000 StackOverflow answers all wrong 000");
            Console.WriteLine("https://stackoverflow.com/questions/15801655/difference-between-two-lists-preserving-duplicates"); 
			Console.WriteLine("000 I don't post on StackOverflow because of the idiotic moderators and comments 000");
            Console.WriteLine(); 

            //https://stackoverflow.com/questions/15801655/difference-between-two-lists-preserving-duplicates
            //Dustin Kingen
            List<string> M = new List<string>() { "1", "1", "2", "3", "4", "5", "9", "7", "7", "7" };
            List<string> N = new List<string>() { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };
         
            var ListDiffs = M.RemoveRange(N);
            sw.Stop();  
            Console.WriteLine("RemoveRage Result = " + string.Join(",", ListDiffs) + " in " + sw.ElapsedTicks + " ticks.");

            sw.Reset();
            sw.Start();
			
            //https://stackoverflow.com/questions/15801655/difference-between-two-lists-preserving-duplicates
            //Ani
            List<string> list2 = new List<string>() { "1", "1", "2", "3", "4", "5", "9", "7", "7", "7" };
            List<string> list1 = new List<string>() { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };
            
            var lookup2 = list2.ToLookup(str => str);

            var result = from str in list1
                         group str by str into strGroup
                         let missingCount
                              = Math.Max(0, strGroup.Count() - lookup2[strGroup.Key].Count())
                         from missingStr in strGroup.Take(missingCount)
                         select missingStr;
            sw.Stop();
            Console.WriteLine("Lookup Result = " + string.Join(",", result) + " in " + sw.ElapsedTicks + " ticks.");
            sw.Reset();
            sw.Start();
			
            List<string> AA = new List<string>() { "1", "1", "2", "3", "4", "5", "9", "7", "7", "7" };
            List<string> BB = new List<string>() { "1", "1", "2", "2", "3", "4", "5", "5", "5", "5" };
         
            //https://stackoverflow.com/questions/15801655/difference-between-two-lists-preserving-duplicates
            //Sergey Kalinichenko 
            var diffLinq = AA.Select(e => new { Key = e, Val = 1 })
                    .Concat(BB.Select(e => new { Key = e, Val = -1 }))
                    .GroupBy(e => e.Key, e => e.Val)
                    .SelectMany(g => Enumerable.Repeat(g.Key, Math.Max(0, g.Sum())))
                    .ToList();

            sw.Stop();
            Console.WriteLine("diffLinq Result = " + string.Join(",", diffLinq) + " in " + sw.ElapsedTicks + " ticks.");




	}
}

 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics; 
                    
public static class Program
{
     public static int loopcnt = 0; 
     public static IEnumerable<T> RemoveRange<T>(this IEnumerable<T> source, IEnumerable<T> second)
     {
         var tempList = source.ToList();
​
         foreach (var item in second)
         {
             tempList.Remove(item); //this is cause a copy like RemoveAt below
         }
​
         return tempList;
     }
​
    //RemoveAt for arrays, this is problematic copy and shrink time consuming - see my next post on how to fix this!
    public static void RemoveAt<T>(ref T[] arr, int index)
    {
        for (int a = index; a < arr.Length - 1; a++)