Windows Forms

public class MyCollectionEditor : CollectionEditor
{
        private Type m_itemType = null;

        public MyCollectionEditor(Type type)
                : base(type)
        {
                m_itemType = type;
        }

        protected override CollectionForm CreateCollectionForm()
        {
                Button buttonLoadItem = new Button();
                buttonLoadItem.Text = "Load from DB";
                buttonLoadItem.Click += new EventHandler(ButtonLoadItem_Click);

                m_collectionForm = base.CreateCollectionForm();

                TableLayoutPanel panel1 = m_collectionForm.Controls[0] as TableLayoutPanel;
                TableLayoutPanel panel2 = panel1.Controls[1] as TableLayoutPanel;
                panel2.Controls.Add(buttonLoadItem);

                return m_collectionForm;
        }

        private void ButtonLoadItem_Click(object sender, EventArgs e)
        {
                if (m_itemType.Equals(typeof(MyCustomCollection)))
                {                               
                        MyCustomItem item = ...load from DB...

                        //definition: SetItems(object editValue, object[] value);
                        SetItems( -> what goes here?! <- , new object[] { item });
                }
        }
}