>>***Combo.Gotfocus event
>>Thisform.Button.Tag="A"
>>***Combo.Lostfocus event
>>Thisform.Button.Tag="B"
>>***Button.Click event
>>if this.tag="A"
>>return
>>endif
>
>
>Edward this seems to be a great idea but it did not work for me

Yeah, it seems that you're correct. You may toggle button.enabled property, and this will definitely work, but I admit that this is not the best interface.