HOW TO：使用 Windows ReadFile 函式 (C# 程式設計手冊)
更新：2007 年 11 月
此範例透過讀取和顯示文字檔示範 Windows ReadFile 函式。ReadFile 函式需要使用 unsafe 程式碼，因為必須要有指標做為參數。
傳遞到 Read 函式的位元組陣列為 Managed 型別。這表示 Common Language Runtime (CLR) 記憶體回收行程可依所需重新配置陣列使用的記憶體。為了避免這一點，fixed 會用來取得對記憶體的指標並加以標記，如此一來記憶體回收行程就不會移動它。在 fixed 區塊的結尾，記憶體會自動回到遵從記憶體回收行程的移動。
這項功能稱為「宣告式固定」(Declarative Pinning)。固定的好處在於虛耗的空間很少，除非記憶體回收發生在 fixed 區塊內，但實際上會發生的機率很低。不過，固定可能在全域記憶體回收執行時，導致某些不想要的副作用。記憶體回收行程壓縮記憶體的功能會大幅度受到固定緩衝區的限制。因此，應盡量避免使用固定。
範例
class FileReader
{
const uint GENERIC_READ = 0x80000000;
const uint OPEN_EXISTING = 3;
System.IntPtr handle;
[System.Runtime.InteropServices.DllImport("kernel32", SetLastError = true, ThrowOnUnmappableChar = true, CharSet = System.Runtime.InteropServices.CharSet.Unicode)]
static extern unsafe System.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
);
[System.Runtime.InteropServices.DllImport("kernel32", SetLastError = true)]
static extern unsafe bool ReadFile
(
System.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
);
[System.Runtime.InteropServices.DllImport("kernel32", SetLastError = true)]
static extern unsafe bool CloseHandle
(
System.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 != System.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()
{
return CloseHandle(handle);
}
}
class Test
{
static int Main(string[] args)
{
if (args.Length != 1)
{
System.Console.WriteLine("Usage : ReadFile <FileName>");
return 1;
}
if (!System.IO.File.Exists(args[0]))
{
System.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.
System.Text.ASCIIEncoding Encoding = new System.Text.ASCIIEncoding();
int bytesRead;
do
{
bytesRead = fr.Read(buffer, 0, buffer.Length);
string content = Encoding.GetString(buffer, 0, bytesRead);
System.Console.Write("{0}", content);
}
while (bytesRead > 0);
fr.Close();
return 0;
}
else
{
System.Console.WriteLine("Failed to open requested file");
return 1;
}
}
}