Question

I open a file in C# with FileStream, and I got the file handle number with this line:

IntPtr file_handle = fs.SafeFileHandle.DangerousGetHandle();

Now I want to pass this handle to C++ code and use this handle value to access the file. Is this possible? How to open a file with merely a file handle in C++?

Thanks.

Update

I use C# to P/Invoke into a C++ Win32 DLL(not a COM DLL). I open the file in C# as FileStream, and pass the handle to the C++. Here is some of my code in the C++ DLL:

extern "C" __declspec(dllexport)void read_file(HANDLE file_handle)
{
    char buffer[64];
    ::printf("\nfile = %d\n",file_handle);

    if(::ReadFile(file_handle,buffer,32,NULL,NULL))
    {
        for(int i=0;i<32;i++)
            cout<<buffer[i]<<endl;
    }
    else
        cout<<"error"<<endl;
}

And here is my C# code:

    [DllImport("...",EntryPoint = "read_file", CharSet = CharSet.Auto)]
    public static extern void read_file(IntPtr file_handle_arg);

But I get this error:

Unhandled Exception: System.AccessViolationException: Attempted to read or write
 protected memory. This is often an indication that other memory is corrupt.

Thanks.

Answer 1

You can use win32 calls, the same way the filestream/file constructors do (via p/invoke).

Cracking it open in .NET Reflector, it looks like it is using this function:

[DllImport("kernel32.dll", CharSet=CharSet.Auto, SetLastError=true)]
private static extern SafeFileHandle CreateFile(
  string lpFileName,
  int dwDesiredAccess,
  FileShare dwShareMode,
  SECURITY_ATTRIBUTES securityAttrs,
  FileMode dwCreationDisposition,
  int dwFlagsAndAttributes,
  IntPtr hTemplateFile);

Here is an official reference:

http://msdn.microsoft.com/en-us/library/aa363858(VS.85).aspx

This is just to open the file, though, as you asked when you said:

How to open a file with merely a file handle in C++

If you want to read an already open file, you might have more trouble. I’m not sure. You might be able to use this function:

[DllImport("kernel32.dll", SetLastError=true)]
internal static extern unsafe int ReadFile(
  SafeFileHandle handle,
  byte* bytes,
  int numBytesToRead,
  IntPtr numBytesRead_mustBeZero,
  NativeOverlapped* overlapped
);

http://msdn.microsoft.com/en-us/library/aa365467(v=VS.85).aspx