Implementing a lasso drawing technique

Here's one approach to drawing a lasso rectangle using Delphi:

1. In the OnMouseDown event for the form that you are 'lasso-ing' controls on:

bMarquee := True;
ptOrigin := Point(X,Y) ;
ptMove := Point(X,Y) ;

Pen.Color := clBlack;
Pen.Width := 1;
Pen.Style := psDash;
Brush.Style := bsClear;

DrawMarquee(ptOrigin, ptMove, pmNotXor ) ;

2. In the OnMouseMove event for the form…

if bMarquee = True then begin
  DrawMarquee(ptOrigin, ptMove, pmNotXor) ;
  DrawMarquee(ptOrigin, Point(X,Y), pmNotXor) ;
  ptMove := Point(X, Y) ;
  Canvas.Pen.Mode := pmCopy;
end;

3. In the OnMouseUp event for the form…

if bMarquee = True then begin
  bMarquee := False;
  DrawMarquee(ptOrigin, Point(X,Y), pmNotXor) ;
  ptMove := Point(X,Y) ;
end;

4. The DrawMarquee procedure…

procedure myForm.DrawMarquee
   (mStart, mStop : TPoint; AMode : TPenMode) ;
begin
  Canvas.Pen.Mode := AMode;
  Canvas.Rectangle(mStart.X,mStart.Y,mStop.X,mStop.Y) ;
end;