I would like to do a validate on my submit button and cancel the upload if there is a problem, how can I do this?

My submit code is something like this:

     protected void submitButton_Click(object sender, System.EventArgs e)
    {

         Page.Validate();
         if (!Page.IsValid)
         {

            // Display a non valid message here

             ... <- Would like to cancel the upload here

  I think this executes after the upload, is there a method called before the upload happens?  Any idea if there is a cancel upload call?

Mark.