ex-22-12

Programming Csharp Source Code(代码) Programming Csharp Source Code

// Example 22-12: Using pointers in a C# program

using System;
using System.Runtime.InteropServices;
using System.Text;

class APIFileReader
{
   [DllImport("kernel32", SetLastError=true)]
   static extern unsafe int CreateFile(
      string filename,
      uint desiredAccess,
      uint shareMode,
      uint attributes,   
      uint creationDisposition,
      uint flagsAndAttributes,
      uint templateFile);

   [DllImport("kernel32", SetLastError=true)]
   static extern unsafe bool ReadFile(
      int hFile,
      void* lpBuffer, 
      int nBytesToRead,
      int* nBytesRead, 
      int overlapped);

   // constructor opens an existing file
   // and sets the file handle member 
   public APIFileReader(string filename)
   {
      fileHandle = CreateFile(
         filename,      // filename
         GenericRead,   // desiredAccess 
         UseDefault,    // shareMode
         UseDefault,    // attributes
         OpenExisting,  // creationDisposition 
         UseDefault,    // flagsAndAttributes
         UseDefault);   // templateFile
   }

   public unsafe int Read(byte[] buffer, int index, int count) 
   {
      int bytesRead = 0;
      fixed (byte* bytePointer = buffer) 
      {
         ReadFile(
            fileHandle,             // hfile
            bytePointer + index,    // lpBuffer
            count,                  // nBytesToRead
            &bytesRead,             // nBytesRead
            0);                     // overlapped
      }
      return bytesRead;
   }

   const uint GenericRead = 0x80000000;
   const uint OpenExisting = 3;
   const uint UseDefault = 0;
   int fileHandle;
}

class Test
{
   public static void Main()
   {
      // create an instance of the APIFileReader, 
      // pass in the name of an existing file
      APIFileReader fileReader = 
        new APIFileReader("myTestFile.txt");

      // create a buffer and an ASCII coder      
      const int BuffSize = 128;
      byte[] buffer = new byte[BuffSize];
      ASCIIEncoding asciiEncoder = new ASCIIEncoding();

      // read the file into the buffer and display to console
      while (fileReader.Read(buffer, 0, BuffSize) != 0)
      {
         Console.Write("{0}", asciiEncoder.GetString(buffer));
      }
   }
}