title: ErrorMessage and ValidationSummary in asp.net (web forms)
body: <p>i'm beginning with web forms in asp.net and i just want a simple example of how can i show a error message on my aspx page. The intention, is to make run as follows:</p>
  
  <ul>
  <li>The user fill 2 fields</li>
  <li>User submit the form</li>
  <li>The 2 fields are compared on the server</li>
  <li>If the two aren't equals, show the same page with the classical red text on the side of the field</li>
  </ul>
  
  <p>It's really a very simple situation, but it's just to know how to work properly with the ErrorMessage and ValidationSummary... </p>
  
  <p>I'm very grateful for any help!</p>
  
  <p>P.S.: I'm using C#</p>