using System;
using System.IO;

namespace Vbaccelerator.Components.Algorithms
{
   /// <summary>
   /// Calculates a 32bit Cyclic Redundancy Checksum (CRC) using the
   /// same polynomial used by Zip.
   /// </summary>
   public class CRC32
   {      
      private UInt32[] crc32Table;
      private const int BUFFER_SIZE = 1024;

      /// <summary>
      /// Returns the CRC32 for the specified stream.
      /// </summary>
      /// <param name="stream">The stream to calculate the CRC32 for</param>
      /// <returns>An unsigned integer containing the CRC32
       calculation</returns>
      public UInt32 GetCrc32(System.IO.Stream stream)
      {
         unchecked
         {
            UInt32 crc32Result;
            crc32Result = 0xFFFFFFFF;
            byte[] buffer = new byte[BUFFER_SIZE];
            int readSize = BUFFER_SIZE;

            int count = stream.Read(buffer, 0, readSize);
            while (count > 0)
            {
               for (int i = 0; i < count; i++)
               {
                  crc32Result = ((crc32Result) >> 8) ^ crc32Table[(buffer[i]) ^
                   ((crc32Result) & 0x000000FF)];
               }
               count = stream.Read(buffer, 0, readSize);
            }
            
            return ~crc32Result;
         }
      }

      /// <summary>
      /// Construct an instance of the CRC32 class, pre-initialising the table
      /// for speed of lookup.
      /// </summary>
      public CRC32()
      {
         unchecked
         {
            // This is the official polynomial used by CRC32 in PKZip.
            // Often the polynomial is shown reversed as 0x04C11DB7.
            UInt32 dwPolynomial = 0xEDB88320;
            UInt32 i, j;

            crc32Table = new UInt32[256];

            UInt32 dwCrc;
            for(i = 0; i < 256; i++)
            {
               dwCrc = i;
               for(j = 8; j > 0; j--)
               {
                  if ((dwCrc & 1)==1)
                  {
                     dwCrc = (dwCrc >> 1) ^ dwPolynomial;
                  }
                  else
                  {
                     dwCrc >>= 1;
                  }
               }
               crc32Table[i] = dwCrc;
            }
         }
      }
   }
}

http://www.vbaccelerator.com/home/net/code/libraries/CRC32/Crc32_zip_CRC32_CRC32_cs.asp

Calculate a CRC32 like the one used in zip files

C#