>>I have another problem i.e: I can add a combol-box control into a grid column but I don't know how to delete the existing textbox control.

1) Select the textbox in thes list of controls
2) Select with the mouse the textbox control (image) in the column of the grid
3) Supr the textbox, the column is empty
4) Add de combol-box in the column

Bye..