title: Fluent Validation ensuring a list has at least one item with property value of somevalue
body: <p>Assume I have the following viewmodel:</p>
  
  <pre><code>public class TaskViewModel{
    public MTask Task {get;set;}
    public List&lt;DocIdentifier&gt; Documents {get;set;}
    .....
  }
  
  public class DocIdentifier{
    public string DocID {get;set;}
    public bool Selected {get;set;}
  }
  </code></pre>
  
  <p>And here's the <code>Fluent Validation</code> validator I use:</p>
  
  <pre><code>public class TaskValidator : AbstractValidator&lt;TaskViewModel&gt;{
     public TaskValidator{
  
     }
  }
  </code></pre>
  
  <p>How can I make sure that at least one DocIdentifier object in the list <code>Documents</code> has its <code>Selected</code> property value <code>True</code>?</p>