Problem using WdfRequestGetParameters (WDF newbie)

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Hello to all and sorry for my english ;-)

I'm trying to write a kmdf driver for an usb device; in my EvtDeviceAdd
callback I use this code:

WDFQUEUE _ioctl_queue;
WDF_IO_QUEUE_CONFIG _ioctl_queue_config;

_queue_config.EvtIoRead = MyEvtIoReadWrite;
_queue_config.EvtIoWrite = MyEvtIoReadWrite;
_queue_config.EvtIoDeviceControl=MyEvtIoDeviceControl;
_status=WdfIoQueueCreate(_device, &_queue_config,
WDF_NO_OBJECT_ATTRIBUTES, &_queue);

In a test application, once retreived device handle, I write a request
(7 bytes) and read the response (512 bytes) using WriteFile and
ReadFile calls.
Both I/O request are managed by the callback that I've registered
(MyEvtIoReadWrite).
In this function I attempt to determine the request type to perform
different operations:

WDF_REQUEST_PARAMETERS _request_parameters;
WDFMEMORY _memory;

WdfRequestGetParameters(Request, &_request_parameters);
if (_request_parameters.Type==WdfRequestTypeWrite){
_status=WdfRequestRetrieveInputMemory(Request, &_memory);
....
}
else{
_status=WdfRequestRetrieveOutputMemory(Request, &_memory);
....
}

While the first call (corresponding to user-mode WriteFile) works fine
(7 bytes written), at the second call (corresponding to user-mode
ReadFile), _request_parameters.Type member is always set to
WdfRequestTypeWrite (but Length parameter is now 512) and the
subsequential call to WdfRequestRetrieveInputMemory fails due to
STATUS_INVALID_DEVICE_REQUEST (obviously).

What is wrong?
Thanks and greetings from Rome

.

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