Answering my question may require some familiarity with printer
drivers in addition to interop. I'm trying to implement some interop
code between a C# application and our UniDriver printer driver. I'm
having two problems really:

1) I need to set some data in the OEMDEVMODE portion of the private
devmode section. Using the GetHdevmode method of the PrinterSettings
class and Marshall.PtrToStructure, I can successfully retrieve the
standard DevMode structure data. This is possible because the
definition of this structure is known. I also know the definition of
our OEMDEVMODE structure. The problem is that there is a UniDrv
devmode structure between the two. The definition of this structure is
not known. The standard devmode structure has two fields that hold the
length of the standard devmode and the length of the unidrv devmode
PLUS our OEMDEVMODE. Is it possible using managed C# code to retrieve
the standard devmode, do some pointer math, and then retrieve just the
OEMDEVMODE?

2) Our C# application needs to recognize when we're printing using our
printer driver as opposed to any other printer. I planned to do this
by using the GetHdevnames() method provided by the PrinterSettings
class and looking at the port name. However, unlike the success I had
in retrieving the standard devmode using the GetHdevmode method, the
data retrieved using the GetHdevnames doesn't look right. Here's the
code:

    [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)]
    struct DEVNAMES
    {
        public short dnDriverOffset;
        public short dnDeviceOffset;
        public short dnOutputOffset;
        public short dnDefault;
    }

    // Get a handle to the devnames for this printer
    IntPtr hDevNames = pd.PrinterSettings.GetHdevnames();
    IntPtr pDevNames = GlobalLock( hDevNames );
    DEVNAMES devnames = (DEVNAMES)Marshal.PtrToStructure( pDevNames,
typeof(DEVNAMES) );
    GlobalUnlock( hDevNames );

The problem is that the structure members are supposed to be offsets
from the start of the structure to the beginning of null terminated
strings. The value I get for the first offset is 4! The next is 5.
This would point into the structure itself. Even if the offsets looked
correct, I'd have to know how to use them (more pointer math?) to
retrieve the strings.

Would appreciate any help you can give me.

Thanks,
Tom Demler