Grouping child objects in WPF TreeView

You can achieve this effect by binding the ItemsSource on your HierarchicalDataTempalate using an IValueConverter. This converter is simply does the following:

public class MyConverter : IValueConverter
{
  public object Convert(object value, ...)
  {
    return
      from item in (IEnumerable<MyItem>)value
      group item by item.Type into g
      select new { Type = g.Key, Items = g }
  }
  ...
}

Now your HierarchcialDataTemplate can be as follows:

<HierarchicalDataTemplate ItemsSource="{Binding SomePath, Converter={x:Static local:MyConverter}">

  <HierarchicalDataTemplate.ItemTemplate>
    <HierarchicalDataTemplate
      ItemsSource="{Binding Items}"
      TargetType="{x:Type local:MyItem}"

      ItemTemplate="{StaticResource MyItemTemplate}">
         <!-- may omit ItemTemplate in prior line to use implicit template -->

      <TextBlock Text="{Binding Type}" /> <!-- Header for type -->

    </HierarchicalDataTemplate>
  </HierarchicalDataTemplate.ItemTemplate>

  <!-- header for "Object Group #1" -->

</HierarchicalDataTemplate>

Take a look at this article by Mr. Sumi. I'm sure it will help you.