Code:

using System;
using System.Collections.Generic;
using LibUsbDotNet;
using LibUsbDotNet.Info;
using LibUsbDotNet.Main;

namespace USBTest
{
    /// <summary>
    /// The general class to obtain the TinyCon devices.
    /// </summary>
    static class TinyConUSBClass
    {
        const int VENDORID = 0x16C0;
        const int PRODUCTID = 0x05DC;

        public static List<TinyConDeviceClass> TinyConDevices = new List<TinyConDeviceClass>();

        /// <summary>
        /// Gets all the connected TinyCon devices and store them in TinyConDevices
        /// </summary>
        /// <returns>void</returns>
        public static void GetUSBDevices()
        {
            //Find devices matching the vendor and product ID
            UsbDeviceFinder USBFinder = new UsbDeviceFinder(VENDORID, PRODUCTID);
            //Using the registry create a list of those devices
            
            UsbRegDeviceList USBRegistryDevices = UsbDevice.AllDevices.FindAll(USBFinder);

            foreach (UsbRegistry USBReg in USBRegistryDevices)
            {
               TinyConDevices.Add(new TinyConDeviceClass(USBReg));
            }
        }

        /// <summary>
        /// Closes all the TinyCon devices and exits the USB driver
        /// </summary>
        public static void ExitUSB()
        {
            foreach (TinyConDeviceClass TinyCon in TinyConDevices)
                TinyCon.Close();

            UsbDevice.Exit();
        }
    }

    /// <summary>
    /// Using my own USB Request type as libusbdotnet's version seems to be incomplete
    /// </summary>
    public enum USBRequestType{
        RECIPIENT_DEVICE = 0,
        RECIPIENT_INTERFACE = 1,
        RECIPIENT_ENDPOINT = 2,
        RECIPIENT_OTHER = 3,
        TYPE_STANDARD = 0,
        TYPE_CLASS = 32,
        TYPE_VENDOR = 64,
        TRANSFER_DIRECTION_OUT = 0,
        TRANSFER_DIRECTION_IN = 128
    }

    /// <summary>
    /// The class for individual TinyCon devices.
    /// </summary>
    class TinyConDeviceClass
    {
        private UsbRegistry _USBRegistry;
        private UsbDevice _USBDevice = null;
        private char[] _USBBuffer = new char[64];

        private enum TinyConRequest
        {
            LED_STATE = 1
        }

        /// <summary>
        /// Is the TinyCon device open. (Read-only)
        /// </summary>
        public bool IsOpen
        {
            get
            {
                if (_USBDevice == null)
                    return false;
                return _USBDevice.IsOpen;
            }
        }

        /// <summary>
        /// Read TinyCon serial number. (Read-only)
        /// </summary>
        public string GetSerialNumber
        {
            get
            {
                if (_USBDevice == null)
                    return "Device not open.";
                return _USBDevice.Info.SerialString;
            }
        }

        public TinyConDeviceClass(UsbRegistry USBRegistry)
        {
            _USBRegistry = USBRegistry;
        }

        /// <summary>
        /// Open the TinyCon device.
        /// </summary>
        /// <returns>bool</returns>
        public bool Open()
        {
            if (!_USBRegistry.Open(out _USBDevice))
                return false;

            if (_USBDevice == null)
                return false;

            IUsbDevice wholeUsbDevice = _USBDevice as IUsbDevice;
            if (!ReferenceEquals(wholeUsbDevice, null))
            {
                // This is a "whole" USB device. Before it can be used, 
                // the desired configuration and interface must be selected.

                // Select config #1
                wholeUsbDevice.SetConfiguration(1);

                // Claim interface #0.
                wholeUsbDevice.ClaimInterface(0);
            }

            return true;
        }

        /// <summary>
        /// Set the LED1 State.
        /// </summary>
        /// <param name="State">bool</param>
        /// <returns>bool</returns>
        public bool LEDState(bool State)
        {
            //object and length can be null/0 if no data packet is involved.
            UsbSetupPacket SetupPacket = new UsbSetupPacket((byte)(USBRequestType.RECIPIENT_DEVICE | USBRequestType.TYPE_VENDOR), (byte)TinyConRequest.LED_STATE, (short)(State ? 1 : 0), 0, 0);
            int LengthTransfered = 0;

            if (_USBDevice == null)
                return false;

            return _USBDevice.ControlTransfer(ref SetupPacket, null, 0, out LengthTransfered);
        }

        /// <summary>
        /// Closes the TinyCon device.
        /// </summary>
        public void Close()
        {
            if (_USBDevice != null)
            {
                _USBDevice.Close();
                _USBDevice = null;
            }
        }
    }
}

Code:

TinyConUSBClass.GetUSBDevices();

if (TinyConUSBClass.TinyConDevices.Count > 0)
{
    TinyConUSBClass.TinyConDevices[0].Open();
     Console.WriteLine("Serial Number: " + TinyConUSBClass.TinyConDevices[0].GetSerialNumber);
     TinyConUSBClass.TinyConDevices[0].LEDState(true);
}
//No need to call  TinyConUSBClass.TinyConDevices[0].Close(); as ExitUSB closes all devices
TinyConUSBClass.ExitUSB();

Code:

USB_PUBLIC uchar usbFunctionSetup(uchar data[8]) {
   usbRequest_t *rq = (void *)data; // cast data to correct type
   switch(rq->bRequest) { // custom command is in the bRequest field
      case USB_LED_ON:
      PORTB |= (1<<3);         // turn LED on (ATTiny45 PB3)
      return 0;
      case USB_LED_OFF:
      PORTB &= ~(1<<3);                    // turn LED off (ATTiny45 PB3)
      return 0;
   }
   return 0; // should not get here
}

Code:

static inline void usbProcessRx(uchar *data, uchar len)
{
usbRequest_t    *rq = (void *)data;

/* usbRxToken can be:
 * 0x2d 00101101 (USBPID_SETUP for setup data)
 * 0xe1 11100001 (USBPID_OUT: data phase of setup transfer)
 * 0...0x0f for OUT on endpoint X
 */
    DBG2(0x10 + (usbRxToken & 0xf), data, len + 2); /* SETUP=1d, SETUP-DATA=11, OUTx=1x */
    USB_RX_USER_HOOK(data, len)
#if USB_CFG_IMPLEMENT_FN_WRITEOUT
    if(usbRxToken < 0x10){  /* OUT to endpoint != 0: endpoint number in usbRxToken */
        usbFunctionWriteOut(data, len);
        return;
    }
#endif
    if(usbRxToken == (uchar)USBPID_SETUP){
        if(len != 8)    /* Setup size must be always 8 bytes. Ignore otherwise. */
            return;
        usbMsgLen_t replyLen;
        usbTxBuf[0] = USBPID_DATA0;         /* initialize data toggling */
        usbTxLen = USBPID_NAK;              /* abort pending transmit */
        usbMsgFlags = 0;
        uchar type = rq->bmRequestType & USBRQ_TYPE_MASK;
        if(type != USBRQ_TYPE_STANDARD){    /* standard requests are handled by driver */
            replyLen = usbFunctionSetup(data);
        }else{
            replyLen = usbDriverSetup(rq);
        }