C# Definition:

  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Pack = 1)]
  struct NativeDeviceInterfaceDetailData
  {
    public int    size;
    public char   devicePath;
  }

C# Alternative Definition:

  [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)]
  struct SP_DEVICE_INTERFACE_DETAIL_DATA
  {
    public int cbSize;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = BUFFER_SIZE)]
    public string DevicePath;
  }

VB.net Definition:

<StructLayout(LayoutKind.Sequential)> _
Public Structure SP_DEVICE_INTERFACE_DETAIL_DATA
     Public cbSize As UInt32
     <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=256)> _
     Public DevicePath As String
End Structure

Members:

size, Size of the structure, in bytes.

devicePath, NULL-terminated string that specifies the device path.

Notes:

The cbSize parameter should be the size of the DWORD plus the first character of the string

SP_DEVICE_INTERFACE_DETAIL_DATA didd = new SP_DEVICE_INTERFACE_DETAIL_DATA();
didd.cbSize = 4 + Marshal.SystemDefaultCharSize; // trust me :)

Notes 2:

The alternative definition is not recommended. The cbSize parameter is different on various operating systems. For Win7 64bit the field must be set to value returned by

didd.cbSize = Marshal.SizeOf(typeof(SP_DEVICE_INTERFACE_DETAIL_DATA))

of the first definition, which is 8 and this is the only value accepted by the OS. However when Vista 32bit is used the size must be set to

didd.cbSize = sizeof(int) + Marshal.SystemDefaultCharSize;

as it was stated in the notes above.

To use the structure it must be allocated on the system global heap to create space for the device path in the structure. The definition is header only which has room for null character only (text terminator).

Example:

unsafe internal string GetDeviceInterfaceDetail(ref DeviceInterfaceData.NativeDeviceInterfaceData devIfaceData)
{
  NativeDeviceInterfaceDetailData* devIfaceDetailData = null;

  try
  {
   int devIfaceDetailDataSize = 0;

   for (;;)
   {
    int result = SetupDiGetDeviceInterfaceDetail(
     _devInfoList, ref devIfaceData, ref *devIfaceDetailData, devIfaceDetailDataSize, out devIfaceDetailDataSize, IntPtr.Zero);

    if (result == 0 && Marshal.GetLastWin32Error() != ErrorInsufficientBuffer)
     throw new Win32Exception();

    if (result != 0)
     break;

    Marshal.FreeHGlobal((IntPtr)devIfaceDetailData);
    devIfaceDetailData = (NativeDeviceInterfaceDetailData*)Marshal.AllocHGlobal(devIfaceDetailDataSize);

    devIfaceDetailData->size = Marshal.SizeOf(
     typeof(NativeDeviceInterfaceDetailData));
   }

   return Marshal.PtrToStringAuto(new IntPtr(&devIfaceDetailData->devicePath));

  }
  finally
  {
   Marshal.FreeHGlobal((IntPtr)devIfaceDetailData);
  }
}