In File1 I created a class with 3 strings. I created another class with a public arraylist. I want this arraylist to be dynamic and the object it contains are the class with the 3 strings. I can access the members of the class in the file but not in a separate file.

file1

public class SensorCollection
    {
        public string ipAddress;
        public string portNumber;   
        public string physicalLocation;

        public DetectorCollection(string ipAddr, string portNum, string loc)
        {
            this.ipAddress = ipAddr;
            this.portNumber = portNum;
            this.physicalLocation = loc;

        }
    }
    public class SensorCollectionArray
    {
        public System.Collections.ArrayList SensorArrayList;
    }

...
System.Collections.ArrayList DetectorArrayList = new System.Collections.ArrayList(); 
...
  DetectorArrayList.Add(new DetectorCollection(ipAddress, portNum, str));

So I can fill the array of classes but can't access it in a separate file. File 2

    AdvancedSettingsForm.SensorCollectionArray mainDetectorCollectionArray;
    System.Collections.ArrayList arrList;

• c#
• arraylist

If you create a SensorCollectionArray like this:

SensorCollectionArray mySCA = new SensorCollectionArray();

Then you can access it's ArrayList like this (for example, to add an item):

mySCA.SensorArrayList.Add(mySensorCollection);

Note however, that in the code you've posted, you didn't include a constructor for the SensorCollectionArray, so the SensorArrayList will be null after instantiation. So you can either set it to a separately instantiated ArrayList, or you can create the ArrayList within your SensorCollectionArray class.

Final note: You might want to look into the generic List(of T) class if you want to create a strongly typed collection

Not entirely sure what're attempting to do, but I assume it's something like the below. Presumably, you're creating a collection of your sensors because you want to apply some rules of some kind before storing it to the collection.

"Is this a good sensor? It is? Add it to the collection!"

Otherwise, you could just use a

List<Sensor> mySensors;

and not really use a class that'll essentially doing the same things. Aside from that, like it's been mentioned there's not really a reason to use ArrayList. As Marc points out here, the most compelling reason to use ArrayList is if you're using .NET 1.1; otherwise, you should use the generic List collection and all the great things it does for you.

//Sensor.cs
public class Sensor
{
    public string Ip{ get; set; }
    public string Port{ get; set; }
    public string PhysicalLocation{ get; set }

    public Sensor(string ipAddr, string portNum, string loc)
    {
        Ip= ipAddr;
        Port= portNum;
        PhysicalLocation= loc;
    }
}

//SensorCollection.cs
public class SensorCollection
{
    private List<Sensor> sensors;

    public Sensor this[int i]
    {
        get { return this.sensors[i]; }
        set { this.sensors[i] = value; }
    }

    public IEnumerable<Sensor> Sensors
    {
        get{ return this.sensors; }
    }

    public SensorCollection()
    {
        sensors = new List<Sensor>();
    }

    public SensorCollection(string ip, string port, string location) : this()
    {
        this.sensors.Add(new Sensor(ip, port, location));
    }

    public SensorCollection(Sensor sensor) : this()
    {
        this.sensors.Add(sensor);
    }

    public void AddSensor(Sensor sensor)
    {
        //Determine whether or not to add it
        this.sensors.Add(sensor);
    }

    public void RemoveSensor(Sensor sensor)
    {
        if (sensors.Contains(sensor))
            sensors.Remove(sensor);
    }
}

Edit

How do I access the ipaddress of each sensor in my dynamically created list of classes?

var mySensors = new SensorCollection();
mySensors.AddSensor(new Sensor("1.1.1.1", "123", "Home"));
mySensors.AddSensor(new Sensor("9.9.9.9", "123", "Work"));

foreach(Sensor s in mySensors.Sensors)
    Console.WriteLine(s.Ip);

I can not seem to access the members of the class in another file

Make sure they're in the same namespace, or that you include a "using" statement that includes the namespace of your classes you create.