title: Validating a group of controllers together using ONE validator in ASP. NET
body: <p>I have 3 DropDownLists, Day, Month, Year, so simply, date of birth.</p>
  
  <p>I need to validate these 3 together, means, if I selected day = 2, and month = 3, but I didn't select a Year (default selected value = null), I need this validator to tell me that the DOB is not fully set.</p>
  
  <p>How can I link these 3 DropDownLists together? </p>