Everything works fine, the combo is populated, I can select from the list etc.

If by editing I clear the text of the combo, as if a wanted no value for the field (which is perfectly leagal in my case),

then I get an InvalidCastException inside the descriptor of the property at:

public override void SetValue(Object component, object Value)
{
      ((CustomerBO)component).RefererID = (System.Guid)Value;
}

The Value object on the right of the assignment is DBNull, and this is why the cast fails. How can I overcome this?

Thanks

This thing is giving me a bit of a hard time

I have not spent a lot of time in DevExpress lately, but I believe there is an option that when the field is cleared to reset the bound value to DbNull.  That sounds like what is happening.  If you do have the need for the field to be reset to DbNull, then you need to add the DbNull support to the field through the BOMapper customization for that field.  You can return an alternate value on Null or use a Nullable generic.  If you make a change through the BO Mapper, be sure to rebuild the partial class so that the Null logic gets built into the BO.

But from the sounds of it I think that you just want to prevent the textbox from setting the value to DbNull.