title: LINQy way to check if any objects in a more than one collection have the same property value
body: <p>I have three collections of the same model, only changing their names.</p>
  
  <p>want to check the three lists so that:</p>
  
  <p>1) If an element in either list is already in another burst an error<br>
  2) When I try to validate the Any does not accept null value.<br>
  How do they accept that the string is null?</p>
  
  <p>but I can not.</p>
  
  <p>where I am going wrong?</p>
  
  <p><code>item.Requeridos</code> > <code>ICollection&lt;MeetingUser&gt;</code> <br>
  <code>item.Informados</code> > <code>ICollection&lt;MeetingUser&gt;</code> <br> 
  <code>item.Opcionais</code> > <code>ICollection&lt;MeetingUser&gt;</code> <br></p>
  
  <pre><code>public static ValidationResult validaUsuariosNaLista(Reuniao item)
  {
      var requeridos = item.Requeridos.Select(x =&gt; string.IsNullOrWhiteSpace(x.Login)).Any();
      var informados = item.Informados.Select(x =&gt; string.IsNullOrWhiteSpace(x.Login)).Any();
      var opcionais  = item.Opcionais .Select(x =&gt; string.IsNullOrWhiteSpace(x.Login)).Any();
  
      if (item.Requeridos.Count() + item.Informados.Count() + item.Opcionais.Count() &gt; item.Requeridos.Concat(item.Informados).Concat(item.Opcionais).Distinct().Count())
      {
          return new ValidationResult(Resources.Validations.sameUserInOtherList);
      }
  
      return ValidationResult.Success;
  }
  </code></pre>