Thaks, Erik.
I tried to solve this problem this way:
if i change:

combo.valid method:
this.value=this.displayvalue

combo.value_assign:
LPARAMETERS vNewVal
*To do: Modify this routine for the Assign method
THIS.Value = m.vNewVal
this.displayvalue=this.value

it´s working fine. BUT when I use the control in a grid with Sparse=.f. extrange behaviours occurs...