title: Bind to Count of List where Typeof
body: <p>I know how to Bind to Count, but how do I do it, if I only want the count where type is Product</p>
  
  <pre><code>&lt;TextBlock Text="{Binding Items.Count}" /&gt;
  Items = new ObservableCollection&lt;object&gt;();
  </code></pre>
  
  <p>I tried with a Property, But I am having problem keeping it in sync when Items are being added or removed.</p>
  
  <pre><code>    public int ProductCount
              {
                  get
                  {
                      return Items.Cast&lt;object&gt;().Count(item =&gt; item.GetType() == typeof (ProductViewModel));
                  }
              }
  </code></pre>