The following code uses ClientRect to find and draw a line from the upper left to the lower right of the current control. Select a TControl in the ListBox list. This example assumes that the current object is a descendant of TControl and has a public Canvas property.
if (ListBox1.Items.Objects[Index] is TListBox) then begin myListBox:= (ListBox1.Items.Objects[Index] as TListBox); with myListBox.ClientRect do begin // Make sure you are using the canvas of the TControl, and not the from's canvas. myListBox.Canvas.MoveTo(Left, Top); myListBox.Canvas.LineTo(Right, Bottom); end; myListBox:= nil; end;