title: Linq GroupBy with each null value as a group
body: <p>I have an object with a nullable int property "GroupId".</p>
  
  <p>With a List of this object, I would like to do a GroupBy on this "GroupId". But if I do it, all the null values will form a group.</p>
  
  <p>Example :</p>
  
  <p>Object 1 : GroupId : NULL</p>
  
  <p>Object 2 : GroupId : NULL</p>
  
  <p>Object 3 : GroupId : 1</p>
  
  <p>Object 4 : GroupId : 1</p>
  
  <p>Object 5 : GroupId : 2</p>
  
  <p>Object 6 : GroupId : 2</p>
  
  <pre><code>MyList.GroupBy(f =&gt; f.GroupId, key =&gt; new {Object = key});
  </code></pre>
  
  <p>I will get 3 groups.</p>
  
  <p>How can I get 4 groups instead ? A group for each NULL value...</p>