Object in c# error

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I don't consider myself an inexperienced programmer nor a brilliant one however this little problem has stumped me. Envisage the simple program at the bottom outlining situation.

A brief explanation of the program:
1. When main form is opened it declares a 'window' and puts a new 'ObjectToTest' in the constructor

2. Upon opening the new modal form it assigns the object handed to the form to 2 variables named 'OriginalObject' and 'ModifiedObject'

3. I then modify 'ModifiedObject' by adding 9 to its list

4.When the modal form returns and shows the 2 MessageBoxs they both show the same value 9. Yet there source variable created is of different values.




My Question: Why is the MessageBox not showing the correct variable?:confused:
(My guess is this is something to do with pointers, something i'm not experienced with)



Thankyou to the people who clear this up for me :)
---------------------------------------------------
Main Form Code:

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            string OriginalString = null;
            string ModifiedString = null;
            window TestWindow = new window(new ObjectToTest());
            TestWindow.ShowDialog(this);
            foreach (int OriginalNumber in TestWindow.GetOriginal().List)
            {
                OriginalString += " " + OriginalNumber;
            }
            foreach (int ModifiedNumber in TestWindow.GetModified().List)
            {
                ModifiedString += " " + ModifiedNumber;
            }
            MessageBox.Show("Original String = " + OriginalString);
            MessageBox.Show("Modified String = " + ModifiedString);
        }
    }

My own testing class (Simple class with one list property):

public class ObjectToTest
    {
        private List<int> Something = new List<int>();
        public List<int> List
        {
            set
            {
                Something = value;
            }
            get
            {
                return Something;
            }
        }
    }

An empty form dialog box I created:

public partial class window : Form
    {
        ObjectToTest OriginalObject = new ObjectToTest();
        ObjectToTest ModifiedObject = new ObjectToTest();

        public window()
        {
            InitializeComponent();
        }

        public window(ObjectToTest ObjectToEdit)
        {
            InitializeComponent();

            OriginalObject = ObjectToEdit;
            ModifiedObject = ObjectToEdit;

            //Make 'ModifiedObject' Different
            ModifiedObject.List.Add(9);
        }

        public ObjectToTest GetOriginal()
        {
            return OriginalObject;
        }

        public ObjectToTest GetModified()
        {
            return ModifiedObject;
        }

    }