Occurs when the control is validated.
Use this event to write code that performs additional validation on the control.
This event occurs for Edit, CheckBox, ComboBox, DateTime, ListBox, and RadioButton controls when the user moves between pages or clicks OK in the form.
The Page object also fires an OnValidate event when the form's OK button is pressed, after all Control objects' OnValidate events fire on the page. This allows for additional page validation.