C#

TextBox tbx = new TextBox { Text = "wtf" };

this.Controls.Add(tbx);

var c1  = this.Controls.Count;

tbx.Dispose();

var c2 = this.Controls.Count; // yep, one less

bool isnull = tbx == null;  // => false

=/// <summary>Releases all resources used by the <see cref="T:System.ComponentModel.Component" />.</summary>
public void Dispose()
{
  this.Dispose(true);
  GC.SuppressFinalize((object) this);
}

Key point: Controls added to a ControlCollection and docked 'Top followed by the use of 'BringToFront on the Control will be added beneath other controls. When one of these controls is removed/deleted, the other Controls will rearrange so that the order is maintained.

Quote:

a.1. set event handlers for the ToDo close-button Click, and checkbox CheckStateChanged, events.

public ToDo()
{
    InitializeComponent();
    CbxDone = cbxDone;
    LblID = lblId;
    BtnClose = btnClose;
    TxtBx = textBox1;
}

public CheckBox CbxDone { set; get; }
public Label LblID { set; get; }
public Button BtnClose { set; get; }
public TextBox TxtBx { set; get; }

private void btnAdd_Click(object sender, EventArgs e)
{
    ToDo todo = new ToDo();
    currentToDos.Add(todo);  // a list of all hosted controls
    currentToDo = todo;      // current hosted vontrol

    todo.LblID.Text = (idCount).ToString();
    idCount++;

    // attach the EventHandlers
    todo.Enter += TodoOnEnter;
    todo.Leave += TodoOnLeave;

    todo.BtnClose.Click += BtnCloseOnClick;

    todo.CbxDone.CheckStateChanged += CbxDoneOnCheckStateChanged;
    
    todopanel.Controls.Add(todo);
    todo.Dock = DockStyle.Top;
    todo.BringToFront();

    this.ActiveControl = todo;
}

currentToDos.Remove(currentToDo);
this.todopanel.Controls.Remove(currentToDo);
currentToDo.Dispose();

this.Invalidate();

using System.Windows.Forms;

namespace ToDo
{
    public partial class Item : UserControl
    {
        public Panel ItemPanel { set; get; }
        public CheckBox DoneCheckBox { set; get; }
        public Button DeleteButton { set; get; }
        public TextBox UserTextBox { set; get; }

        public Item()
        {
            InitializeComponent();

            ItemPanel = itemPanel;
            DoneCheckBox = doneCheckBox;
            DeleteButton = deleteButton;
            UserTextBox = userTextBox;
        }
    }
}

List<Item> itemList = new List<Item>();
Item crtItem;

void AddItem()
{
    if (inputTextBox.Text != "")
    {
        Item item = new Item();
        itemList.Add(item);
        crtItem = item;
        item.Dock = DockStyle.Top;
        item.UserTextBox.Text = inputTextBox.Text;
        item.Enter += Item_Enter;
        item.Leave += Item_Leave;
        item.DoneCheckBox.CheckedChanged += DoneCheckBox_CheckedChanged;
        item.DeleteButton.Click += DeleteButton_Click;
        itemPanel.Controls.Add(item);
        item.BringToFront();

        inputTextBox.Text = "";
        itemsLabel.Text = itemList.Count + " item(s)";
    }

    inputTextBox.Focus();
}

private void Item_Enter(object sender, EventArgs e)
{
    crtItem.ItemPanel.BackColor = Color.FromArgb(200, 233, 253);
    crtItem.UserTextBox.BackColor = Color.FromArgb(200, 233, 253);
}

private void Item_Leave(object sender, EventArgs e)
{
    crtItem.ItemPanel.BackColor = Color.FromArgb(240, 240, 240);
    crtItem.UserTextBox.BackColor = Color.FromArgb(240, 240, 240);
}

private void DoneCheckBox_CheckedChanged(object sender, EventArgs e)
{
    if (crtItem.DoneCheckBox.Checked)
    {
        crtItem.UserTextBox.ForeColor = Color.FromArgb(150, 150, 150);
        crtItem.UserTextBox.Font = new Font("Segoe UI", 18F, FontStyle.Strikeout, GraphicsUnit.Point, 0);
    }
    else
    {
        crtItem.UserTextBox.ForeColor = Color.FromArgb(0, 0, 0);
        crtItem.UserTextBox.Font = new Font("Segoe UI", 18F, FontStyle.Regular, GraphicsUnit.Point, 0);
    }
}

private void DeleteButton_Click(object sender, EventArgs e)
{
    itemList.Remove(crtItem);
    itemPanel.Controls.Remove(crtItem);
    crtItem.Dispose();

    Invalidate();
}

alin1 wrote:

How do I update its value when I click a random Item usercontrol?

private void Item_Enter(object sender, EventArgs e)
{
    crtItem = (Item)sender;
    crtItem.ItemPanel.BackColor = Color.FromArgb(200, 233, 253);
    crtItem.UserTextBox.BackColor = Color.FromArgb(200, 233, 253);
}

private void Item_Leave(object sender, EventArgs e)
{
    crtItem = (Item)sender;
    crtItem.ItemPanel.BackColor = Color.FromArgb(240, 240, 240);
    crtItem.UserTextBox.BackColor = Color.FromArgb(240, 240, 240);
}

Quote:

Are there any circumstances where you would want to keep track of not only the current active 'Item, but, the previous one ?

SELECT * 
FROM INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE ccu
WHERE ccu.CONSTRAINT_NAME = 'UserIDfk'