title: Check null value in a list using linq
body: <p>I have a list <code>List&lt;OfferComparison&gt; Comparison</code>. I want to 
  check if all the items have <code>Value == null</code> in an <code>if</code> condition.</p>
  
  <p>How can I do it with linq?</p>
  
  <pre><code>public class OfferComparison : BaseModel
  {
      public string Name { get; set; }
      public string Value { get; set; }
      public bool Valid { get; set; }
  }
  </code></pre>