C#調用API示例

摘自MSDN，這是一段C#調用Windows系統API函數的示例代碼。
在自定義的FileReader類中，導入了CreateFile, ReadFile, CloseHandle三個API函數，並新建了Open，Read，Close方法來應用它們。
在編譯時需用unsafe選項，因爲用到了C#中所謂的指針。
FileReader類在窗體程序、控制檯程序、Web程序中皆可使用，不過這只是一個示例，個人可自由發揮，使用其他API，來實現某些“高級”功能。

// readfile.cs
// compile with: /unsafe
// arguments: readfile.cs
 
// Use the program to read and display a text file.
using System;
using System.Runtime.InteropServices;
using System.Text;
 
class FileReader
{
      const uint GENERIC_READ = 0x80000000;
      const uint OPEN_EXISTING = 3;
      IntPtr handle;
 
      [DllImport("kernel32", SetLastError=true)]
      static extern unsafe IntPtr CreateFile(
            string FileName,                    // file name
            uint DesiredAccess,                 // access mode
            uint ShareMode,                     // share mode
            uint SecurityAttributes,            // Security Attributes
            uint CreationDisposition,           // how to create
            uint FlagsAndAttributes,            // file attributes
            int hTemplateFile                   // handle to template file
            );
 
       [DllImport("kernel32", SetLastError=true)]
      static extern unsafe bool ReadFile(
            IntPtr hFile,                       // handle to file
            void* pBuffer,                      // data buffer
            int NumberOfBytesToRead,            // number of bytes to read
            int* pNumberOfBytesRead,            // number of bytes read
            int Overlapped                      // overlapped buffer
            );
 
      [DllImport("kernel32", SetLastError=true)]
      static extern unsafe bool CloseHandle(
            IntPtr hObject   // handle to object
            );
     
      public bool Open(string FileName)
      {
            // open the existing file for reading         
            handle = CreateFile(
                  FileName,
                  GENERIC_READ,
                  0,
                  0,
                  OPEN_EXISTING,
                  0,
                  0);
     
            if (handle != IntPtr.Zero)
                  return true;
            else
                  return false;
      }
 
      public unsafe int Read(byte[] buffer, int index, int count)
      {
            int n = 0;
            fixed (byte* p = buffer)
            {
                  if (!ReadFile(handle, p + index, count, &n, 0))
                        return 0;
            }
            return n;
      }
 
      public bool Close()
      {
            // close file handle
            return CloseHandle(handle);
      }
}
 
class Test
{
      public static int Main(string[] args)
      {
            if (args.Length != 1)
            {
                  Console.WriteLine("Usage : ReadFile <FileName>");
                  return 1;
            }
           
            if (! System.IO.File.Exists(args[0]))
            {
                  Console.WriteLine("File " + args[0] + " not found.");
                  return 1;
            }
 
            byte[] buffer = new byte[128];
            FileReader fr = new FileReader();
           
            if (fr.Open(args[0]))
            {
                 
                  // Assume that an ASCII file is being read
                  ASCIIEncoding Encoding = new ASCIIEncoding();
                 
                  int bytesRead;
                  do
                  {
                        bytesRead = fr.Read(buffer, 0, buffer.Length);
                        //string content = Encoding.GetString(buffer,0,bytesRead);
                        string content = System.Text.Encoding.Default.GetString(buffer, 0, bytesRead);
                        Console.Write("{0}", content);
                  }
                  while ( bytesRead > 0);
                 
                  fr.Close();
                  return 0;
            }
            else
            {
                  Console.WriteLine("Failed to open requested file");
                  return 1;
            }
      }
}

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