public class ProjectClass
{
    public List<ItemType> ItemTypes { get; set; }

    public List<Item> Items { get; set; }
}

public class ItemType
{
    public int Id { get; set; }
    public string TypeName { get; set; }
}

public class Item
{
    private int _itemTypeId;

    public int ItemTypeId
    {
        get { return _itemTypeId; }
        set {
            _itemTypeId = value;
            // How to get TypeName to assign to TypeName property of this list item ???
        }
    }
    public string TypeName { get; set; } // ????
}