NetDuino - fun with rotary encoders

I'm playing with a Netduino microcontroller with a view to making a digital readout for my lathe (using a linear encoder from machine-dro.co.uk). So I experimented with a couple of rotary controllers I had lying around which were pretty flaky. After quite a bit of googling and poking around, I found the following code gave me the most satisfactory results, so I thought I'd post it in case anybody else found it handy (feel free to suggest improvements):

public class RotaryEncoder

    {

        private InterruptPort PortA;

        private InterruptPort PortB;

        private uint prevA = 0;

        private uint prevB = 0;

        private uint prevPort = 0;

        public int Position{get; private set;}

        

        public RotaryEncoder(Cpu.Pin pinA, Cpu.Pin pinB)

        {

            PortA = new InterruptPort(

                    pinA, 

                    false, 

                    Port.ResistorMode.PullUp, 

                    Port.InterruptMode.InterruptEdgeBoth);

            PortB = new InterruptPort(

                    pinB, 

                    false, 

                    Port.ResistorMode.PullUp, 

                    Port.InterruptMode.InterruptEdgeBoth);

            PortA.OnInterrupt += OnInterrupt;

            PortB.OnInterrupt += OnInterrupt;

        }

        void OnInterrupt(uint port, uint state, DateTime time)

        {

            if (port == prevPort)

                return;

            prevPort = port;

            if (((uint)PortA.Id) == port){

                if (state > 0 && prevB > 0)

                    --Position;

                prevA = state;

            }

            else{

                if (prevA == 0 && state == 0)

                    ++Position;

                prevB = state;

            }    

        }  

    }