Composite HID device, how to get the right interface?

[hlshih]

composite HID device (generic + keyboard), generic use interface 0, keyboard use interface 1.
I use
_device = HidDevices.Enumerate(VendorId, ProductId).FirstOrDefault();
to get the device, and use
_device.Write(buffer);
to write data to generic device, but generic device fail to get data. What's wrong?
If only generic device only (not composite USB device), HID generic get data success.