title: Adding missing items to collection using LINQ
body: <p>I have two collections from different classes that have these two commpon properties:</p>
  
  <pre><code>public class Class1
  {
      public string ItemID {get;set;}
      public int sales {get;set;}
  }
  
  public class Class2 
  {
      public string ItemID {get;set;}
  }
  </code></pre>
  
  <p>Now what happens here I have two collections with these types of classes like this:</p>
  
  <pre><code>List&lt;Class1&gt; list1; // has items that actually have sales...
  
  List&lt;Class1&gt; list2; // has all items that have or don't have sales
  </code></pre>
  
  <p><code>list1</code> is missing items that don't have sales at all. And what I would like to do here is somehow compare these two lists by the <code>itemID</code>, and if the <code>itemID</code> is missing, then I'm gonna add the missing item to <code>list1</code> with setting property <code>sales</code> to 0 manually.</p>
  
  <p>And when the missing item is found, then i'd like to do this:</p>
  
  <pre><code>list1.Add(new Class1
  {
      Sales = 0,
      ItemID = ItemID,
  });
  </code></pre>
  
  <p>Or some other way to add the missing items and setting their property <code>sales</code> to 0...</p>
  
  <p>Can someone help me out?</p>
  
  <p>Edit:</p>
  
  <p>Guys I think i hve more explaining to do... So basically if the ItemID from Class2 list isn't present in Class1 list, then it's added to Class1 list with property "Sales" set to 0....</p>