title: WPF IDataErrorInfo multiple field validation
body: <p>What is a good way to integrate multiple field validation with IDataErrorInfo?</p>
  
  <p>Let say that I have a dialog with 3 textboxes for ftp information</p>
  
  <ul>
  <li>URL</li>
  <li>Username</li>
  <li>Password</li>
  </ul>
  
  <p>I have put the Required attribute on the fields (assume a normal TextBox for the password).</p>
  
  <p>I validate the ftp connection when the user press "OK". At the moment I show a dialog but it would be nice if I could trigger the Validation error style on ftp connection errors.</p>
  
  <p>I have looked at Validation.MarkInvalid but don't understand how to use it.</p>
  
  <pre><code>var be = GetBindingExpression(xamlURLField);
  Validation.MarkInvalid(be, new ValidationError(-- WhatValidationRuleToPutHere --, be, "Can't connect to ftp", null)
  </code></pre>