How to make PropertyGrid to fire PropertyValueChanged event, when one of it's “by link” values changed?

using System.Windows.Forms;
using System.Collections.Generic;

namespace TestPropGrid
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            TestObject to = new TestObject();
            propertyGrid1.SelectedObject = to;
        }

        private void propertyGrid1_PropertyValueChanged(object s, PropertyValueChangedEventArgs e)
        {
            MessageBox.Show(this, "save time!", "occurs only when property really changed", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
    }

    public class TestSubObject
    {
        public int SomeField { get; set; }
    }

    public class TestObject
    {
        public List<TestSubObject> TSO { get; set; } //event will not fire when items added or removed or elements edited
        public int TestField { get; set; } //event fires when needed

        public TestObject()
        {
            this.TSO = new List<TestSubObject>();
        }
    }
}

This is test code to show the problem. When I add members or edit their SomeField, the propertyGrid1_PropertyValueChanged doesn't fires, while I want to use it to save the item.



I understand that event isn't fired, because pointer to TSO actually didn't changed, but what I should do to know if it's time save editing item after such edits?