Discovering controls beneath a programmatically drawn rectangle

Based on frogleg's sampel app, I was able to get the pseudo-highlighting of the controls working. Here's the pertinent code:

// Declare a rectangle and a point:
Rectangle describedRect;
Point startingPoint;
private bool _IsSelecting = false; // <- already had this

// In the MouseDown event:
_IsSelecting = true;
startingPoint = e.Location;

// In the MouseMove event:
if (!_IsSelecting)
 return;
describedRect = Rectangle.FromLTRB(startingPoint.X, startingPoint.Y, e.X, e.Y);

// In the MouseUp event:
if (!_IsSelecting)
 return;
HighlightAllTextBoxValsBetweenPoints();
_IsSelecting = false;

private void HighlightAllTextBoxValsBetweenPoints() {
 foreach (Control ctrl in tableLayoutPanelGreatGooglyMoogly.Controls) {
  var tb = ctrl as TextBox;
  if (tb == null)
   continue;

  if (describedRect.IntersectsWith(tb.Bounds)) {
   tb.BackColor = PSEUDO_HIGHLIGHT_COLOR;
  }
 }
}