Error checking in a web form
<p>Is there somewhere to see examples of error checking.  I have a web form with several controls.  Two of the controls are critical to updating a database and also for the reporting in a gridview.  </p>

<p>I am not looking for extremely complex checking, just something to stop the user before the submitting an update to the database.  For example, I have a calendar where the user needs to select a date.  If they fail to select a date, I would like to not process the update and instead return a warning like "Are you nuts, enter a date!"</p>

<p>TIA</p>
