how to do this transient mark trick

[matthias <pfemat@web.de>]

Hi there,

I have

  '(transient-mark-mode t)

in my custom-set-variables and for each of the four arrow-keys of my 
keyboard i have a function of the following kind in my .emacs file and i 
bind the according function to the corresponding arrow-key:

(defun set-mark-and-move-point-right ()
   "sets mark if mark is not set and move point one line up"
   (interactive)
   (when (not (region-active-p))
     (call-interactively 'set-mark-command)
     )
   (forward-char 1))

region-active-p looks like that

(defun region-active-p ()
   "Say whether the region is active or not."
   (and (boundp 'transient-mark-mode)
        transient-mark-mode
        (boundp 'mark-active)
        mark-active))

i achieve the desired behavior that the region is activated and 
colorized by transient-mark-mode when i press shift-<some_arrow>. 
However i have the feeling that this is not a clean way to achieve this 
since using these functions I a can also

press shift-<arrow>
release shift already
press some <arrow>s and region is still colorized


call for help is like this: Can it be done better? Is it possible to 
de-activate the region when shift is released?

thanks for your help.

matthias