If you use the EpiCombo control, you can also set its style to DropDownList through custom code, so that it can only be selected through the drop-down list and the user cannot enter anything.
Exp:
add a EpiCombo
Set RetrieveOnActivate to fales
Set Staic data you need
Add custom code
epiComboC1.DropDownStyle = Infragistics.Win.UltraWinGrid.UltraComboStyle.DropDownList;
Save your custm form to test it.
