title: Problem with TreeNodeCollection.ContainsKey()
body: <p>I'm creating a plugin for an application which works with a concept of families.
  Each Family belongs to a FamilyCategory and each Family contains FamilySymbols.
  A nice tree structure like that:</p>
  
  <ul>
  <li>FamilyCategory (Doors)
  <ul>
  <li>Family (External Doors)
  <ul>
  <li>FamilySymbol (Door 2000x1000)</li>
  <li>FamilySymbol (Door 2000x900)</li>
  </ul></li>
  <li>Family (Garage Doors)
  <ul>
  <li>FamilySymbol (Door 2000x2000)</li>
  <li>FamilySymbol (Door 2100x2000)</li>
  </ul></li>
  </ul></li>
  <li>FamilyCategory (Windows)
  <ul>
  <li>Family (Single Windows)
  <ul>
  <li>FamilySymbol (Window 1000x1400)</li>
  <li>FamilySymbol (Window 800x1400)</li>
  </ul></li>
  <li>Family (Double Windows)
  <ul>
  <li>FamilySymbol (Window 2000x1400)</li>
  <li>FamilySymbol (Window 2100x1400)</li>
  </ul></li>
  </ul></li>
  </ul>
  
  <p>Now I'm trying to build a TreeView representing that structure. I have a list of Family objects and each of them has a FamilyCategory property. I'm trying to determine if a TreeNode with FamilyCategory name already exists and if it does I'm trying to add the Family to that node. If a node for that category doesn't exist I create a new one and add the family there.
  Unfortunately the code below always evaluates categoryExists as false.</p>
  
  <pre><code>foreach (Family family in families)
  {
      string familyCategoryName = family.FamilyCategory.Name;
  
      bool categoryExists = treeView.Nodes.ContainsKey(familyCategoryName);
  
      if (categoryExists)
      {
          categoryNode = treeView.Nodes[familyCategoryName];
      }
      else
      {
          categoryNode = new TreeNode(familyCategoryName);
          treeView.Nodes.Add(categoryNode);
      }
  
      TreeNode familyNode = new TreeNode(family.Name);
  
      categoryNode.Nodes.Add(familyNode);
  
      foreach (FamilySymbol familySymbol in family.Symbols)
      {
          familyNode.Nodes.Add(familySymbol.Name);
      }
  }
  </code></pre>
  
  <p>What am I doing wrong?</p>