title: PropertyChanged for indexer property
body: <p>I have a class with an indexer property, with a string key:</p>
  
  <pre><code>public class IndexerProvider {
      public object this[string key] {
          get
          {
              return ...
          }
          set
          {
              ...
          }
      }
  
      ...
  }
  </code></pre>
  
  <p>I bind to an instance of this class in WPF, using indexer notation:</p>
  
  <pre><code>&lt;TextBox Text="{Binding [IndexerKeyThingy]}"&gt;
  </code></pre>
  
  <p>That works fine, but I want to raise a <code>PropertyChanged</code> event when one of the indexer values changes. I tried raising it with a property name of "[keyname]" (i.e. including [] around the name of the key), but that doesn't seem to work. I don't get binding errors in my output window whatsoever.</p>
  
  <p>I can't use CollectionChangedEvent, because the index is not integer based. And technically, the object isn't a collection anyway.</p>
  
  <p>Can I do this, and so, how?</p>