C#

ShowDialog(IWin32Window owner)

this.tabControl1.Controls.Add(this.tabPage2);
this.tabControl1.Controls.Add(this.tabPage1);

protected override void OnControlAdded(ControlEventArgs e)
{
    base.OnControlAdded(e);

    m_Pages.Add(e.Control as TabPage);
}

public class TabControlDesigner : ParentControlDesigner
{
    private DesignerVerbCollection m_Verbs;

    public override void InitializeNewComponent(IDictionary defaultValues)
    {
        base.InitializeNewComponent(defaultValues);

        OnAddTabPage(this, EventArgs.Empty);
        OnAddTabPage(this, EventArgs.Empty); 
    }

    public override DesignerVerbCollection Verbs
    {
        get
        {
            return m_Verbs != null ? m_Verbs : m_Verbs = new DesignerVerbCollection(new DesignerVerb[] { new DesignerVerb("Add Tab", OnAddTabPage) });
        }
    }

    protected void OnAddTabPage(Object sender, EventArgs e)
    {
        IDesignerHost Host = (IDesignerHost)GetService(typeof(IDesignerHost));

        if (Host == null)
            return;

        TabControl Control = Component as TabControl;

        if (Control == null)
            return;

        TabPage newPage = Host.CreateComponent(typeof(TabPage)) as TabPage;

        MemberDescriptor Descriptor = TypeDescriptor.GetProperties(Control)["Controls"];

        RaiseComponentChanging(Descriptor);

        newPage.Text = newPage.Name;
        newPage.Owner = Component as TabControl;
        newPage.BackColor = Color.White;

        Control.Controls.Add(newPage);
                
        RaiseComponentChanged(Descriptor, null, null);
    }

    public override bool CanParent(Control c)
    {
        if (c is TabPage)
            return !Control.Contains(c);
        else
            return false;
    }

    protected override void WndProc(ref Message m)
    { 
        if (m.Msg == 0x201 /*WM_LBUTTONDOWN*/)
        {
            TabControl c = Component as TabControl;

            int lParam = m.LParam.ToInt32();
            Point p = new Point(lParam & 0xffff, lParam >> 0x10);


            for (int i = 0; i < c.Controls.Count; i++)
            {
                 if (c.GetTabRect(i).Contains(p))
                 {
                     c.SelectedIndex = i;
                     return;
                 }
            }
            
        }

        base.WndProc(ref m);
    }

    protected override void OnDragDrop(DragEventArgs e)
    {
        ((IDropTarget)((TabControl)Component).SelectedPage).OnDragDrop(e);
    }

    protected override void OnDragEnter(DragEventArgs e)
    {
        ((IDropTarget)((TabControl)Component).SelectedPage).OnDragEnter(e);
    }

    protected override void OnDragLeave(EventArgs e)
    {
        ((IDropTarget)((TabControl)Component).SelectedPage).OnDragLeave(e);
    }

    protected override void OnDragOver(DragEventArgs e)
    {
        ((IDropTarget)((TabControl)Component).SelectedPage).OnDragOver(e);
    }
}