const int USB_STRING_DESCRIPTOR_TYPE = 3;

The USB_DESCRIPTOR_REQUEST structure is always followed by either a USB_STRING_DESCRIPTOR or USB_DEVICE_DESCRIPTOR structure.

The use of a a zero-length array at the end of the structure requires that you read and write "off the edge" of the structure. This concept is not well supported in C#/VB.Net so you have make sure that you allocate sufficient buffer space to accommodate a the USB_STRING_DESCRIPTOR or USB_DEVICE_DESCRIPTOR structure.

   Request.SetupPacket.wValue = (short)((USB_STRING_DESCRIPTOR_TYPE << 8) + PortDeviceDescriptor.iManufacturer);

     USB_STRING_DESCRIPTOR StringDesc = (USB_STRING_DESCRIPTOR)Marshal.PtrToStructure(ptrStringDesc, typeof(USB_STRING_DESCRIPTOR));

The values for iManufacturer, iProduct, and iSerialNumber are just indexs that are used by the USB_STRING_DESCRIPTOR request

struct USB_STRING_DESCRIPTOR

Structure USB_STRING_DESCRIPTOR

const int USB_STRING_DESCRIPTOR_TYPE = 3;

typedef struct _USB_STRING_DESCRIPTOR {

} USB_STRING_DESCRIPTOR, *PUSB_STRING_DESCRIPTOR;

You don't use an IOCTL call directly with a USB_STRING_DESCRIPTOR structure. Instead you use a USB_DESCRIPTOR_REQUEST "request packet" with IOCTL_USB_GET_DESCRIPTOR_FROM_NODE_CONNECTION. The USB_STRING_DESCRIPTOR structure is returned at the very end of the request packet. The whole idea of "writing off the edge" of a structure is counter to the way C#/VB.Net was designed to work, so you'll have to make sure to allocate sufficient amount of memory to handle both the "request packet" and the USB_STRING_DESCRIPTOR structure.

   Request.SetupPacket.wValue = (short)((USB_STRING_DESCRIPTOR_TYPE << 8) + PortDeviceDescriptor.iManufacturer);

     USB_STRING_DESCRIPTOR StringDesc = (USB_STRING_DESCRIPTOR)Marshal.PtrToStructure(ptrStringDesc, typeof(USB_STRING_DESCRIPTOR));