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I am attempting to connect to a USB GPS device. I can successfully connect to the device if I manually create a file via CreateFile WinApi (using the path specified in Device Manager).

However, when I try to select the device through enumeration I fail @ the SetupDiGetDeviceInterfaceDetail call.

I have C code that works correctly, but my C# translation does not appear to work correctly. I have tried many variations with essentially the same results.

C Code that works

// Get enumerator handle for the specified ClassGuid
HDEVINFO theDevInfo = SetupDiGetClassDevs((GUID*)&GUID_DEVINTERFACE_GRMNUSB, NULL, NULL,
    DIGCF_PRESENT | DIGCF_INTERFACEDEVICE);
SP_DEVICE_INTERFACE_DATA theInterfaceData;
theInterfaceData.cbSize = sizeof(theInterfaceData);
// populate theInterfaceData which contains device class information
if (!SetupDiEnumDeviceInterfaces(theDevInfo, NULL, (GUID*)&GUID_DEVINTERFACE_GRMNUSB, 0, &theInterfaceData) &&
    GetLastError() == ERROR_NO_MORE_ITEMS)
  gHandle = 0;
  return;
// This is normally used to obtain the device path information using theInterfaceData obtained above
bool initialized = SetupDiGetDeviceInterfaceDetail(theDevInfo, &theInterfaceData, NULL, 0, &theBytesReturned, NULL);
// theBytesReturned = 83
theDevDetailData =
(PSP_INTERFACE_DEVICE_DETAIL_DATA)malloc(theBytesReturned);
theDevDetailData->cbSize = sizeof(SP_INTERFACE_DEVICE_DETAIL_DATA);
bool initialized = SetupDiGetDeviceInterfaceDetail(theDevInfo, &theInterfaceData, theDevDetailData, theBytesReturned, NULL, &theDevInfoData);
[DllImport(@"setupapi.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern Boolean SetupDiGetDeviceInterfaceDetail(
    IntPtr hDevInfo,
    ref SP_DEVICE_INTERFACE_DATA deviceInterfaceData,
    IntPtr deviceInterfaceDetailData,
    UInt32 deviceInterfaceDetailDataSize,
    out UInt32 requiredSize,
    IntPtr deviceInfoData
[StructLayout(LayoutKind.Sequential)]
public struct SP_DEVICE_INTERFACE_DATA
    public Int32 cbSize;
    public Guid interfaceClassGuid;
    public Int32 flags;
    private UIntPtr reserved;
// Get enumerator handle for the specified ClassGuid
IntPtr theDevInfo = SetupDiGetClassDevs(ref ClassGuid, (DiGetClassFlags.DIGCF_PRESENT | DiGetClassFlags.DIGCF_DEVICEINTERFACE));
SP_DEVICE_INTERFACE_DATA DevInterfaceData = new SP_DEVICE_INTERFACE_DATA();
DevInterfaceData.cbSize = Marshal.SizeOf(DevInterfaceData);
initialized = SetupDiEnumDeviceInterfaces(theDevInfo, IntPtr.Zero, ref ClassGuid, 0,
        ref DevInterfaceData); 
// I assume The DevInterfaceData is populated correctly as it matches the C Code
// And I've compared the values in memory and they match
uint bytesReturned = 0;
initialized = SetupDiGetDeviceInterfaceDetail(theDevInfo, ref DevInterfaceData, IntPtr.Zero, 0, out bytesReturned, IntPtr.Zero);
// I expect bytesReturned = 83 and initialized = true which is the value that is returned in the C Code
// Instead the value 162 is returned
Congratulations, it is working.  You'll get a Unicode string, it's twice as long.  And a FALSE return is correct.  You just need to call Marshal.GetLastWin32Error() and verify that you got ERROR_INSUFFICIENT_BUFFER.  Your C code is broken, probably because you forgot to initialize theBytesReturned to zero.
                The C code works properly.  It was the C# code that did not work properly.  theBytesReturned is initialized to 0, it's just not shown in the above code as I didn't put the entire function to save space.  I guess the C code returned 83 bytes due to being ASCII, but if the C# code was expecting a UNICODE string, why is it 162 and not 83x2 = 166?
– galford13x
                Feb 12, 2012 at 8:55
                I don't know, post the strings and I'll tell you why.  There really is a problem with the C code, the function must return FALSE and GetLastError() must return ERROR_INSUFFICIENT_BUFFER.  You then know the required buffer size, allocate it and call the function again.
– Hans Passant
                Feb 12, 2012 at 9:15
                Oh yes I agree, the first call does return false, there is a second call that I apparently didn't put that is the actually deviceDetailData read.  I'll edit my post to be correct.  Here is the string @"\\?\usb#vid_091e&pid_0003#5&4a03a84&0&5#{2c9c45c2-8e7d-4c08-a12d-816bbae722c0}"
– galford13x
                Feb 12, 2012 at 9:37
After poking around pinvoke.net, I found the following
// build a Device Interface Detail Data structure
 SP_DEVICE_INTERFACE_DETAIL_DATA didd = new SP_DEVICE_INTERFACE_DETAIL_DATA();
 if (IntPtr.Size == 8) // for 64 bit operating systems
     didd.cbSize = 8; 
 else  didd.cbSize = 4 + Marshal.SystemDefaultCharSize; // for 32 bit systems
Changing this in my code allows me to obtain the DevicePath properly.
Reference for those interested.
http://pinvoke.net/default.aspx/setupapi.SetupDiGetDeviceInterfaceDetail
                No, that corrupts the stack.  Use Marshal.AllocHGlobal() to allocate the buffer with the required size.  The size you got from the first call.
– Hans Passant
                Feb 12, 2012 at 8:25
                why/how is the stack corrupted?  Wont the proper memory be marshaled when I make the Win32api call?
– galford13x
                Feb 12, 2012 at 9:43
                @HansPassant any chance you could explain why the test for IntPtr.Size is necessary and we can't rely on the marshaler to get SizeOf(SP_DEVICE_INTERFACE_DETAIL_DATA) right? I'm guessing it has to do with padding somehow?
– Ohad Schneider
                Jun 21, 2014 at 17:25
                @Ohad - an old quirk in SetupAPI, it used 1 byte packing in the 32-bit version of Windows.  Corrected to 8 byte packing in 64-bit.  It can be done by the marshaller but you'd have to declare two different structs with different Pack property values.
– Hans Passant
                Jun 21, 2014 at 17:47
                @HansPassant Thanks for the quick turnaround! As long as I have to declare two different structs I don't see it as being much better, so I guess I'll stick with this code.
– Ohad Schneider
                Jun 21, 2014 at 17:49
        
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