MD5的作用详见：https://baike.baidu.com/item/MD5/212708?fr=aladdin

public static string GetFileMD5(string filepath)
        {
            FileStream fs = new FileStream(filepath, FileMode.Open, FileAccess.Read, FileShare.Read);
            int bufferSize = 1048576;
            byte[] buff = new byte[bufferSize];
            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            md5.Initialize();
            long offset = 0;
            while (offset < fs.Length)
            {
                long readSize = bufferSize;
                if (offset + readSize > fs.Length)
                    readSize = fs.Length - offset;
                fs.Read(buff, 0, Convert.ToInt32(readSize));
                if (offset + readSize < fs.Length)
                    md5.TransformBlock(buff, 0, Convert.ToInt32(readSize), buff, 0);
                else
                    md5.TransformFinalBlock(buff, 0, Convert.ToInt32(readSize));
                offset += bufferSize;
            }
            if (offset >= fs.Length)
            {
                fs.Close();
                byte[] result = md5.Hash;
                md5.Clear();
                StringBuilder sb = new StringBuilder(32);
                for (int i = 0; i < result.Length; i++)
                    sb.Append(result[i].ToString("X2"));
                return sb.ToString();
            }
            else
            {
                fs.Close();
                return null;
            }
        }

　　filepath：文件的绝对路径