I was surprised as I was working on a project the other day. I had a field that is required under certain conditions. To take care of this I placed a bit of code in the Valid method of the combobox that is bound to this field and didn't give it a second thought. As I was testing my changes, however, I was surprised to find that the valid method was not being fired unless the user changes the field. I was under the impression that the Valid method always fires right before a control looses focus. Well, apparently not. At least not for comboboxes.