[plt-scheme] A question about PLT Graphics Toolkit

hi allI am beginning in scheme language: I don't understant this:

*
;------------------------------------------------------------------------------------
 #lang scheme/gui
 (define f (new frame%
              (label "test") ))


(define c (new canvas%
               (parent f)
               (min-width 200)
               (paint-callback (λ (canvas dc)
                                 (send dc clear)))))


 (define dc (send c get-dc))

(send dc set-background (make-object color% "red"))

(send dc draw-rectangle 0 0 50 50)

(send f show #t )*

;---------------------------------------------------------------------------------------

Why set-background work :  I see my canvas red; but draw-rectangle doesn't
work  there are no rectangle in my canvas.

I try to  delete set-background line but I don't see my rectangle.

so I know if I put  *(send dc draw-rectangle 0 0 50 50)    *in
paint-callback  it will work .

But i want draw a rectangle not directly in my canvas class.


Thanks.
**
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