How to increase performance. File comparer.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

namespace Compare
{
    public class Comparer
    {
        public static List<string> CompareLength(string file, List<string> arr)
        {
            List<string> result = new List<string>();
            foreach (string item in arr)
                if (new FileInfo(item).Length == new FileInfo(file).Length)
                    result.Add(item);
            return result;
        }
        public static void CompareFilesRec(List<string> array)
        {
            List<string> outp = new List<string>();

            int i = 0;
            while (i < array.Count)
            {
                List<string> fromAll = Comparer.CompareLength(array[i], array);
                string chosen = array[i];
                if (fromAll.Count != 0)
                    foreach (var f2 in fromAll)
                    {
                        if (Checksum.CompareHash(chosen, f2))
                        {
                            outp.Add(f2);
                            array.Remove(f2);
                        }
                        else { i++; }

                    }
                Comparer.ShowResults(outp);
                outp.Clear();
            }
        }
        public static void ShowResults(List<string> results)
        {
            if (results.Count >= 2)
            {
                foreach (var element in results)
                {
                    Console.WriteLine(element);
                }
                Console.WriteLine();
            }
        }
    }
}

static public bool CompareHash(string file1, string file2)
{
    long FirstSize = new FileInfo(file1).Length;
    long SecondSize = new FileInfo(file2).Length;

    byte[] buffer1 = new byte[FirstSize];
    byte[] buffer2 = new byte[SecondSize];

    try
    {
        using (var mmf1 = MemoryMappedFile.CreateFromFile(file1, FileMode.OpenOrCreate, null, 0))
        using (var mmf2 = MemoryMappedFile.CreateFromFile(file2, FileMode.OpenOrCreate, null, 0))
        using (var reader1 = mmf1.CreateViewStream())
        using (var reader2 = mmf2.CreateViewStream())
        {
            try
            {
                reader1.Read(buffer1, 0, (int)FirstSize);
                reader2.Read(buffer2, 0, (int)SecondSize);
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error: {0}", ex.Message);
            }


            if (buffer1.Length != buffer2.Length || Crc8.ComputeChecksum(buffer1) != Crc8.ComputeChecksum(buffer2))
                return false;
        }
    }
    catch (IOException) { }
    catch (UnauthorizedAccessException) { }
    return true;
}

namespace Compare
{
    public class Comparer
    {
        public static List<string> CompareLength(string file, List<string> arr)
        {
            List<string> result = new List<string>();
            foreach (string item in arr)
                if (new FileInfo(item).Length == new FileInfo(file).Length && Checksum.CompareHash(file,item))//here I added hash check. To avoid another loop or something.
                    result.Add(item);
            return result;
        }
        public static void CompareFilesRec(List<string> array)
        {
            Dictionary<long, List<string>> di = new Dictionary<long,List<string>>();

            int i = 0;
            while (i < array.Count)
            {
                long size = new FileInfo(array[i]).Length;

                List<string> fromAll = Comparer.CompareLength(array[i], array);

                if (di.ContainsKey(size))
                {
                    fromAll.Clear();
                    array.Remove(array[i]);
                }
                else
                {
                    di.Add(size, fromAll);
                    i++;
                }
            }
            ShowResults(di);
        }
        public static void ShowResults(Dictionary<long, List<string>> dic)
        {
            foreach(var element in dic)
            {
                foreach (var outer in element.Value)
                {
                    Console.WriteLine(outer);
                }
                Console.WriteLine();
            }
        }
    }
}

namespace Compare
{
    public class Checksum
    {
        static public bool CompareHash(string file1, string file2)
        {
            long FirstSize = new FileInfo(file1).Length;
            long SecondSize = new FileInfo(file2).Length;

            SHA256 sha1 = SHA256Managed.Create();
            SHA256 sha2 = SHA256Managed.Create();

            byte[] buffer1 = new byte[FirstSize];
            byte[] buffer2 = new byte[SecondSize];

            try
            {
                using (var mmf1 = MemoryMappedFile.CreateFromFile(file1, FileMode.OpenOrCreate, null, 0))
                using (var mmf2 = MemoryMappedFile.CreateFromFile(file2, FileMode.OpenOrCreate, null, 0))
                using (var reader1 = mmf1.CreateViewStream())
                using (var reader2 = mmf2.CreateViewStream())
                {
                    try
                    {
                        reader1.Read(buffer1, 0, (int)FirstSize);
                        reader2.Read(buffer2, 0, (int)SecondSize);
                    }
                    catch (Exception ex)
                    {
                        Console.WriteLine("Error: {0}", ex.Message);
                    }


                    if (buffer1.Length != buffer2.Length && sha1.ComputeHash(buffer1) != sha2.ComputeHash(buffer2))
                        return false;
                }
            }
            catch (IOException) { }
            catch (UnauthorizedAccessException) { }
            return true;
        }
    }
}