Why is my FileStream constructor throwing an exception for an overlapped HID device 'file' handle?

I'm writing code that needs to read and write (overlapped) from a custom USB device. To test that this works I have written a C# Win32 .Net Framework console app which successfully uses FileStream to talk to the USB device. Here's a snippet from the working code

 var hidHandle = CreateFile(devpath, FileAccess.ReadWrite, FileShare.ReadWrite, IntPtr.Zero, FileMode.Open, FileOptions.Asynchronous, IntPtr.Zero);
 IntPtr lpData;
 if (HidD_GetPreparsedData(hidHandle, out lpData))
 {
     HidCaps devcaps;
     HidP_GetCaps(lpData, out devcaps);
     InputReportByteLength = devcaps.InputReportByteLength; // 65
     OutputReportByteLength = devcaps.OutputReportByteLength; // 65
     fs = new FileStream(hidHandle, FileAccess.ReadWrite, InputReportByteLength, true);
 }

I can then use fs to read and write to the USB device.

When I copy the code over into Unity the call to the FileStream constructor fails and I get this exception thrown:

Invalid handle
at System.IO.FileStream..ctor (IntPtr handle, FileAccess access, Boolean ownsHandle, Int32 bufferSize, Boolean isAsync, Boolean noBuffering) [0x0007d] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/FileStream.cs:97 at System.IO.FileStream..ctor (IntPtr handle, FileAccess access, Boolean ownsHandle, Int32 bufferSize, Boolean isAsync) [0x00000] in :0
at System.IO.FileStream..ctor (Microsoft.Win32.SafeHandles.SafeFileHandle handle, FileAccess access, Int32 bufferSize, Boolean isAsync) [0x00000] in :0
at (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (Microsoft.Win32.SafeHandles.SafeFileHandle,System.IO.FileAccess,int,bool)
at Zanzibar.HIDConnector.HIDDevice.USBThreadWorker () [0x0006a] in C:\\Users\\timregan\\Source\\Zanzibar\\Software\\Playground\\UnityHID\\HidTest\\Assets\\HIDDevice.cs:81

The file handle is not invalid, it has already been used successfully in the code fragment to obtain the InputReportByteLength.

I do not understand why code that works under .Net 2.0 targetting Win32 is failing under Unity. My guess is that it is something to do with overlapping 'file' access. Note that the FileOptions.Asynchronous flag passed into this external call is the FILE_FLAG_OVERLAPPED flag in CreateFile

 [DllImport("kernel32.dll")]
 protected static extern SafeFileHandle CreateFile(String pFileName, FileAccess dwDesiredAccess, FileShare dwShareMode, IntPtr pSecurityAttributes, FileMode dwCreationDisposition, FileOptions dwFlagsAndAttributes, IntPtr hTemplateFile);

Does anyone know how to use FileStream to talk to an overlapped HID device 'file' in Unity? Or how to diagnose why this works in a console app and not under Unity?