The C# ReadFile API

C# Signature:
//1
[DllImport("kernel32.dll", SetLastError = true)]
static extern bool ReadFile(IntPtr hFile, [Out] byte[] lpBuffer,
uint nNumberOfBytesToRead, out uint lpNumberOfBytesRead, IntPtr lpOverlapped);



or

//2
[DllImport("kernel32.dll", SetLastError=true)]
static extern unsafe int ReadFile(IntPtr handle, IntPtr bytes, uint numBytesToRead,
IntPtr numBytesRead, NativeOverlapped* overlapped);



or

//3
[DllImport("kernel32.dll", SetLastError=true)]
static extern bool ReadFile(IntPtr hFile, [Out] byte[] lpBuffer, uint nNumberOfBytesToRead,
out uint lpNumberOfBytesRead, [In] ref System.Threading.NativeOverlapped lpOverlapped);



or

//4

[DllImport("kernel32.dll", SetLastError=true)]
private unsafe static extern bool ReadFile(
int hFile, // handle to file
byte[] lpBuffer, // data buffer
int nNumberOfBytesToRead, // number of bytes to read
ref int lpNumberOfBytesRead, // number of bytes read
int* ptr
// 
// ref OVERLAPPED lpOverlapped // overlapped buffer
);



or

//5

[DllImport(@"kernel32", CharSet = CharSet.Auto, SetLastError = true)]
static extern unsafe bool ReadFile(
SafeFileHandle hFile, // handle to file
byte* pBuffer, // data buffer, should be fixed
int NumberOfBytesToRead, // number of bytes to read
IntPtr pNumberOfBytesRead, // number of bytes read, provide NULL here
NativeOverlapped *lpOverlapped // should be fixed, if not null
);



VB Signature:
 _
Private Shared Function ReadFile(ByVal hFile As IntPtr, ByVal Buffer As IntPtr, _
ByVal nNumberOfBytesToRead As Integer, ByRef lpNumberOfBytesRead As Integer, _
ByRef lpOverlapped As OVERLAPPED) As Integer
End Function



or 

 Friend Shared Function ReadFile( _
ByVal File As SafeFileHandle, _
ByVal Buffer As System.Text.StringBuilder, _
ByVal NumberOfBytesToRead As Integer, _
ByRef NumberOfBytesRead As Integer, _
ByRef Overlapped As System.Threading.NativeOverlapped) As SafeFileHandle
End Function



User-Defined Types:
None.

Notes:
The documentation states that if using unbuffered IO, the memory must be "storage aligned" (aligned to the sector size of the storage device). Either this is not enforced, or managed allocations (specifically byte arrays) are automatically storage aligned because there seems to be no problem using it. Note that storage aligned and page aligned are not the same, and managed allocations are not in general page aligned (required forReadFileScatter).

For more information on unbuffered IO in .NET see:

http://arxiv.org/abs/cs.PF/0502012

Notes:
Regarding the above note, my experience shows that failing to provide a storage-aligned buffer causesReadFile to perform all IO synchronously. I.e. it always blocks until the operation is complete and never returns ERROR_IO_PENDING.

Warning! GetOverlappedResult writes to the address of the buffer specified in the ORIGINAL OPERATION (ie/ReadFile orWriteFile). .NET may move the address of the buffer beforeGetOverlappedResult returns, resulting in a buffer overflow. UseAllocHGlobal andFreeHGlobal or otherwise ensure the buffer is pinned between the two calls. The same applies to the overlapped structure.

Tips & Tricks:
NET 2.0 use SafeFileHandle instead ofIntPtr

Sample Code:
Public Function Read() As stArcnetPacket
Dim dwErrorCode As FarcConstants
Dim erg As stArcnetPacket
Dim dwBytesReceived As Int32
Dim ip As IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(erg))
If ReadFile(hDriver, ip, Marshal.SizeOf(erg), dwBytesReceived, Nothing) Then
erg = Marshal.PtrToStructure(ip, erg.GetType)
ReDim Preserve erg.Data(dwBytesReceived - 6)
Marshal.FreeHGlobal(ip)
Else
dwErrorCode = Marshal.GetLastWin32Error
Marshal.FreeHGlobal(ip)
Throw New System.IO.IOException("Read fails. Errorcode: " & _
dwErrorCode.ToString, New System.ComponentModel.Win32Exception(dwErrorCode))
End If
Return erg
End Function



USED W/ SIGNATURE #4

// This is taken from the USBSharp.cs class
public unsafe byte[] CT_ReadFile(int InputReportByteLength)
{
int BytesRead =0;
byte[] BufBytes = new byte[InputReportByteLength];
if (ReadFile(HidHandle, BufBytes, InputReportByteLength, ref BytesRead, null))
{
byte[] OutBytes = new byte[BytesRead];
Array.Copy(BufBytes, OutBytes, BytesRead);
return OutBytes;
}
else
{
return null;
}
}



Alternative Managed API:
System.IO.FileStream

See Also
WriteFile, WriteFileGather,ReadFileScatter

Documentation

ReadFile on MSDN