SetupDiGetDeviceInterfaceDetail unexplainable error

Question

Hello again folks

This time I have an error which I have been trying again to figure out why it exists. Although the error can be ignored but I'd like to know why it exists in the 1st place.

result = SetupDiGetDeviceInterfaceDetail(deviceInfoSet, ref anInterface, IntPtr.Zero, 0, ref buffersize, IntPtr.Zero);
            if (!result)
            {
                int errCode = Marshal.GetLastWin32Error();
                errorMessage = new Win32Exception(Marshal.GetLastWin32Error()).Message;
                statusLabel.Text += "\n(1)SetupDiGetDeviceInterfaceDetail Error: " + errCode + " => " + errorMessage + ".";
                //break;
            }

This is the 1st call to this function which is just for the purpose of setting the buffersize variable for the second call to the function. I get the printed error message: 122 => The data area passed to a system call is too small. Judging from the error message I figured this must have something to do with the second parameter (ref anInterface) and that I could ignore this for the second pass which turned out to be true; but the error still exists and I'd like to know why before it comes back to bite me in the 'behind' place. The parameter in question is declared and defined thus:

[StructLayout(LayoutKind.Sequential)] // defined here
    public struct SP_DEVICE_INTERFACE_DATA
    {
        public uint cbSize;
        public Guid InterfaceClassGuid;
        public uint Flags;
        public IntPtr Reserved;
    }

anInterface = new SP_DEVICE_INTERFACE_DATA(); // declared here
        anInterface.cbSize = (uint)Marshal.SizeOf(anInterface);
        anInterface.InterfaceClassGuid = Guid.Empty;
        anInterface.Reserved = IntPtr.Zero;
        anInterface.Flags = 0;

I have picked apart the msdn articles and as far as I can tell there's nothing wrong with the code here.

Answer 1

This is supposed to happen by design. If you look here under the comments section:

Using this function to get details about an interface is typically a two-step process:

1) Get the required buffer size. Call SetupDiGetDeviceInterfaceDetail with a NULL DeviceInterfaceDetailData pointer, a DeviceInterfaceDetailDataSize of zero, and a valid RequiredSize variable. In response to such a call, this function returns the required buffer size at RequiredSize and fails with GetLastError returning ERROR INSUFFICIENT BUFFER.
2) Allocate an appropriately sized buffer and call the function again to get the interface details.

If you look up the ERROR INSUFFICIENT BUFFER under the error codes, you will see that it is decimal value 122.