快速地读取大数据量的txt文件，内存映射.

 

  public   class  ReadBigTxt
     {
        ""#region""
        [DllImport("kernel32.dll")]
        public static extern IntPtr CreateFileMapping(IntPtr hFile,
        IntPtr lpFileMappingAttributes, uint flProtect,
        uint dwMaximumSizeHigh,
        uint dwMaximumSizeLow, string lpName);

        [DllImport("kernel32.dll")]
        public static extern IntPtr MapViewOfFile(IntPtr hFileMappingObject, uint
        dwDesiredAccess, uint dwFileOffsetHigh, uint dwFileOffsetLow,
        IntPtr dwNumberOfBytesToMap);

        [DllImport("kernel32.dll")]
        public static extern bool UnmapViewOfFile(IntPtr lpBaseAddress);

        [DllImport("kernel32.dll")]
        public static extern bool CloseHandle(IntPtr hObject);

        [DllImport("kernel32.dll")]
        public static extern IntPtr CreateFile(string lpFileName,
        int dwDesiredAccess, FileShare dwShareMode, IntPtr securityAttrs,
        FileMode dwCreationDisposition, int dwFlagsAndAttributes, IntPtr hTemplateFile);

        [DllImport("kernel32.dll")]
        public static extern uint GetFileSize(IntPtr hFile, IntPtr lpFileSizeHigh);

        public const int GENERIC_READ = -2147483648; //0x80000000 
        public const int GENERIC_WRITE = 0x40000000;
        public const int GENERIC_EXECUTE = 0x20000000;
        public const int GENERIC_ALL = 0x10000000;
        public const int FILE_ATTRIBUTE_NORMAL = 0x80;
        public const int FILE_FLAG_SEQUENTIAL_SCAN = 0x8000000;
        public const int INVALID_HANDLE_VALUE = -1;

        public const int PAGE_NOACCESS = 1;
        public const int PAGE_READONLY = 2;
        public const int PAGE_READWRITE = 4;

        public const int FILE_MAP_COPY = 1;
        public const int FILE_MAP_WRITE = 2;
        public const int FILE_MAP_READ = 4;
        #endregion
        private string m_LogPath = "";
        public ReadBigTxt()
        {

        }
        public string GetFileTxt(string filename)
        {
            string str = "";
            try
            {
                IntPtr vFileHandle = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FileShare.Read | FileShare.Write, IntPtr.Zero, FileMode.Open, FILE_ATTRIBUTE_NORMAL | FILE_FLAG_SEQUENTIAL_SCAN, IntPtr.Zero);
                if (INVALID_HANDLE_VALUE != (int)vFileHandle)
                {
                    IntPtr vMappingHandle = CreateFileMapping(
                    vFileHandle, IntPtr.Zero, PAGE_READWRITE, 0, 0, "temp");
                    if (vMappingHandle != IntPtr.Zero)
                    {
                        IntPtr vHead = MapViewOfFile(vMappingHandle,
                        FILE_MAP_COPY | FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, IntPtr.Zero);
                        if (vHead != IntPtr.Zero)
                        {
                            uint vSize = GetFileSize(vFileHandle, IntPtr.Zero);
                            byte[] nb = new byte[vSize];
                            Marshal.Copy(vHead, nb, 0, nb.Length);
                            str = System.Text.Encoding.GetEncoding("GB2312").GetString(nb);
                            nb = null;
                            UnmapViewOfFile(vHead);
                        }
                        CloseHandle(vMappingHandle);
                    }
                    CloseHandle(vFileHandle);
                }
            }
            catch
            {
                return "";
            }
            return str.Replace("\r", "").Replace("\n", "");

        }
    }