resizing virtualbox machine disk on Ubuntu 12.04 LTS guest

Before starting do a backup and verify it works ....!

• First use vboxmanage  (I want to changemy VM size from 8Go to 15 Go dynamic size): 

VBoxManage modifyhd  yourpath\yourMachine.vdi --resize 15360

you will see something like that  :

0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

• Then add in the VM settings an ubuntu live CD  (containing gparted) and then boot on it. After boot, run gparted. In my case, before extend my partition, I have to move extended partition containing swap. 
• Finally extend your partition.

At the end if you have moved your SWAP partition you'll probably change its UUID (see here). Use the next command to see your current and in use swap UUID :

cat /etc/fstab 

And use 

blkid | grep -i swap 

to see if UUID are the same.... Make correction if needed in fstab.