using System;
 System.IO;
 System.Text;
/// <summary>
/// 提供用于计算指定文件哈希值的方法
<example>例如计算文件的MD5值:
<code>
   String hashMd5=HashHelper.GetMD5("MyFile.txt");
</code>
</example>
例如计算文件的CRC32值:
   String hashCrc32 = HashHelper.GetCRC32("MyFile.txt");
例如计算文件的SHA1值:
   String hashSha1 =HashHelper.GetSHA1("MyFile.txt");
</summary>
public class HashHelper
{
    <summary>
      计算指定文件的MD5值
    </summary>
    <param name="fileName">指定文件的完全限定名称</param>
    <returns>返回值的字符串形式</returns>
    static String GetMD5(String fileName)
    {
        String hashMD5 = String.Empty;
        //检查文件是否存在，如果文件存在则进行计算，否则返回空值
        if (File.Exists(fileName))
        {
            using (FileStream fs = new FileStream(fileName,FileMode.Open,FileAccess.Read))
            {
                计算文件的MD5值
                System.Security.Cryptography.MD5 calculator = System.Security.Cryptography.MD5.Create();
                Byte[] buffer = calculator.ComputeHash(fs);
                calculator.Clear();
                将字节数组转换成十六进制的字符串形式
                StringBuilder stringBuilder =  StringBuilder();
                for (int i = 0; i < buffer.Length; i++)
                {
                    stringBuilder.Append(buffer[i].ToString("x2"));
                }
                hashMD5 = stringBuilder.ToString();
            }关闭文件流
        }结束计算
        return hashMD5;
    }ComputeMD5
 
      计算指定文件的CRC32值
     String GetCRC32(String fileName)
    {
        String hashCRC32 =计算文件的CSC32值
                Crc32 calculator =  Crc32();
                Byte[] buffer =));
                }
                hashCRC32 =关闭文件流
        }
         hashCRC32;
    }ComputeCRC32
 
 
     获取文件的SHA1
    <param name="fileName"></param>
    <returns></returns>
     String GetSHA1(String fileName)
    {
        String hashSHA1 =using (FileStream fileStream = 计算文件的SHA1值
                System.Security.Cryptography.SHA1 calculator = System.Security.Cryptography.SHA1.Create();
                Byte[] buffer = calculator.ComputeHash(fileStream);
                calculator.Clear();
                int bufferIdx = 0; bufferIdx < buffer.Length; bufferIdx++)
                {
                    stringBuilder.Append(buffer[bufferIdx].ToString());
                }
                hashSHA1 = stringBuilder.ToString();
            }else
        {
            Console.Error.WriteLine({0}文件找不到！,fileName);
        }
         hashSHA1;
    }end GetSHA1
}
 提供 CRC32 算法的实现
 Crc32 : System.Security.Cryptography.HashAlgorithm
{
    const UInt32 DefaultPolynomial = 0xedb88320;
    const UInt32 DefaultSeed = 0xffffffffprivate UInt32 hash;
     UInt32 seed;
     UInt32[] table;
    private  UInt32[] defaultTable;
    public Crc32()
    {
        table = InitializeTable(DefaultPolynomial);
        seed = DefaultSeed;
        Initialize();
    }
     Crc32(UInt32 polynomial,UInt32 seed)
    {
        table = InitializeTable(polynomial);
        this.seed = seed;
        Initialize();
    }
    override void Initialize()
    {
        hash = seed;
    }
    protected void HashCore(byte[] buffer,int start,1)">int length)
    {
        hash = CalculateHash(table,hash,buffer,start,length);
    }
    byte[] HashFinal()
    {
        byte[] hashBuffer = UInt32ToBigEndianBytes(~hash);
        this.HashValue = hashBuffer;
         hashBuffer;
    }
    static UInt32 Compute([] buffer)
    {
        return ~CalculateHash(InitializeTable(DefaultPolynomial),DefaultSeed,0static UInt32 Compute(UInt32 seed,seed,1)">static UInt32 Compute(UInt32 polynomial,UInt32 seed,1)">return ~CalculateHash(InitializeTable(polynomial),1)"> UInt32[] InitializeTable(UInt32 polynomial)
    {
        if (polynomial == DefaultPolynomial && defaultTable != null)
        {
             defaultTable;
        }
        UInt32[] createTable = new UInt32[256];
        0; i < 256; i++)
        {
            UInt32 entry = (UInt32)i;
            int j = 0; j < 8; j++)
            {
                if ((entry & 1) == 1)
                    entry = (entry >> 1) ^ polynomial;
                
                    entry = entry >> ;
            }
            createTable[i] = entry;
        }
        if (polynomial == DefaultPolynomial)
        {
            defaultTable = createTable;
        }
         createTable;
    }
    static UInt32 CalculateHash(UInt32[] table,1)"> size)
    {
        UInt32 crc = seed;
        int i = start; i < size; i++unchecked
            {
                crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
            }
        }
         crc;
    }
    [] UInt32ToBigEndianBytes(UInt32 x)
    {
        return new byte[] { (byte)((x >> 24) & 0xff),(16) & 8) & byte)(x & ) };
    }
}end class: Crc32