I'm trying to designing a class and I'm having issues with accessing some of the nested fields and I have some concerns with how multithread safe the whole design is. I would like to know if anyone has a better idea of how this should be designed or if any changes that should be made?

using System;
using System.Collections;

namespace SystemClass
{
public class Program
{
    static void Main(string[] args)
    {
        System system = new System();

        //Seems like an awkward way to access all the members
        dynamic deviceInstance = (((DeviceType)((DeviceGroup)system.deviceGroups[0]).deviceTypes[0]).deviceInstances[0]);
        Boolean checkLocked = deviceInstance.locked;

        //Seems like this method for accessing fields might have problems with multithreading
        foreach (DeviceGroup dg in system.deviceGroups)
        {
            foreach (DeviceType dt in dg.deviceTypes)
            {
                foreach (dynamic di in dt.deviceInstances)
                {
                    checkLocked = di.locked;
                }
            }
        }
    }
}

public class System
{
    public ArrayList deviceGroups = new ArrayList();

    public System()
    {   
        //API called to get names of all the DeviceGroups
        deviceGroups.Add(new DeviceGroup("Motherboard"));
    }
}

public class DeviceGroup
{
    public ArrayList deviceTypes = new ArrayList();

    public DeviceGroup() {}

    public DeviceGroup(string deviceGroupName)
    {
        //API called to get names of all the Devicetypes
        deviceTypes.Add(new DeviceType("Keyboard"));
        deviceTypes.Add(new DeviceType("Mouse"));
    }
}

public class DeviceType
{
    public ArrayList deviceInstances = new ArrayList();
    public bool deviceConnected;

    public DeviceType() {}

    public DeviceType(string DeviceType)
    {
        //API called to get hardwareIDs of all the device instances
        deviceInstances.Add(new Mouse("0001"));
        deviceInstances.Add(new Keyboard("0003"));
        deviceInstances.Add(new Keyboard("0004"));

        //Start thread CheckConnection that updates deviceConnected periodically
    }

    public void CheckConnection()
    {
        //API call to check connection and returns true
        this.deviceConnected = true;
    }
}

public class Keyboard
{
    public string hardwareAddress;
    public bool keypress;
    public bool deviceConnected;

    public Keyboard() {}

    public Keyboard(string hardwareAddress)
    {
        this.hardwareAddress = hardwareAddress;
        //Start thread to update deviceConnected periodically
    }

    public void CheckKeyPress()
    {
        //if API returns true
        this.keypress = true;
    }
}

public class Mouse
{
    public string hardwareAddress;
    public bool click;

    public Mouse() {}

    public Mouse(string hardwareAddress)
    {
        this.hardwareAddress = hardwareAddress;
    }

    public void CheckClick()
    {
        //if API returns true
        this.click = true;
    }
}

}