C# CRC32

CRC32.cs

using System;
using System.Security.Cryptography;
using System.IO;

using System.Net;
using System.Windows.Forms;

public class CRC32 : HashAlgorithm
{
    public const UInt32 DefaultPolynomial = 0xedb88320;
    public const UInt32 DefaultSeed = 0xffffffff;

    private UInt32 hash;
    private UInt32 seed;
    private UInt32[] table;
    private static UInt32[] defaultTable;

    public static string Start(string FilePath)
    {
        if (FilePath == string.Empty) FilePath = Application.ExecutablePath;
        CRC32 crc32 = new CRC32();
        String result = string.Empty;
        try
        {
            using (FileStream fs = File.OpenRead(FilePath))
            {
                foreach (byte b in crc32.ComputeHash(fs))
                {
                    result += b.ToString("x2").ToLower();
                }
                return result;
            }
        }
        catch { return result; }
    }

    public CRC32()
    {
        table = InitializeTable(DefaultPolynomial);
        seed = DefaultSeed;
        Initialize();
    }

    public CRC32(UInt32 polynomial, UInt32 seed)
    {
        table = InitializeTable(polynomial);
        this.seed = seed;
        Initialize();
    }

    public override void Initialize()
    {
        hash = seed;
    }

    protected override void HashCore(byte[] buffer, int start, int length)
    {
        hash = CalculateHash(table, hash, buffer, start, length);
    }

    protected override byte[] HashFinal()
    {
        byte[] hashBuffer = UInt32ToBigEndianBytes(~hash);
        this.HashValue = hashBuffer;
        return hashBuffer;
    }

    public override int HashSize
    {
        get { return 32; }
    }

    public static UInt32 Compute(byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(DefaultPolynomial), DefaultSeed, buffer, 0, buffer.Length);
    }

    public static UInt32 Compute(UInt32 seed, byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(DefaultPolynomial), seed, buffer, 0, buffer.Length);
    }

    public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
    }

    private static UInt32[] InitializeTable(UInt32 polynomial)
    {
        if (polynomial == DefaultPolynomial && defaultTable != null)
            return defaultTable;

        UInt32[] createTable = new UInt32[256];
        for (int i = 0; i < 256; i++)
        {
            UInt32 entry = (UInt32)i;
            for (int j = 0; j < 8; j++)
                if ((entry & 1) == 1)
                    entry = (entry >> 1) ^ polynomial;
                else
                    entry = entry >> 1;
            createTable[i] = entry;
        }

        if (polynomial == DefaultPolynomial)
            defaultTable = createTable;

        return createTable;
    }

    private static UInt32 CalculateHash(UInt32[] table, UInt32 seed, byte[] buffer, int start, int size)
    {
        UInt32 crc = seed;
        for (int i = start; i < size; i++)
            unchecked
            {
                crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
            }
        return crc;
    }

    private byte[] UInt32ToBigEndianBytes(UInt32 x)
    {
        return new byte[] {
   (byte)((x >> 24) & 0xff),
   (byte)((x >> 16) & 0xff),
   (byte)((x >> 8) & 0xff),
   (byte)(x & 0xff)
  };
    }

    public string Get(string FilePath)
    {
        CRC32 crc32 = new CRC32();
        String hash = String.Empty;

        using (FileStream fs = File.Open(FilePath, FileMode.Open))
            foreach (byte b in crc32.ComputeHash(fs)) hash += b.ToString("x2").ToLower();

        return hash;
    }
}