 <p>Handling the TextChanged event should work, however you need to set the DropDownStyle to DropDownList so that the Text property can only be a given value. Then check to see that both comboboxes have values selected. something like this should work:</p> <p>If ComboBox_Ticker.Text &lt;> "" AndAlso DateTime.TryParse(ComboBox_Date.Text, Nothing) Then</p>