<Post Id="0" Caption="aaaaaa" Code="US">
     <Position Caption="Строка 1" SysID="1" Number="5">
           <Post>
                 <Mz Name="A1" Id="11" Value="548231"/>
                 <Mt Name="Б2" Id="0" Value="42323"/>
                 <Mt Name="Б3" Id="1" Value="31231"/>
           </Post>
     </Position>
</Post> 
<Post Id="1" Caption="bbbbbb" Code="US">
     <Position Caption="Строка 1" SysID="1" Number="5">
           <Post>
                 <Mz Name="A1" Id="11" Value="548231"/>
                 <Mt Name="Б2" Id="0" Value="42323"/>
                 <Mt Name="Б3" Id="1" Value="31231"/>
           </Post>
     </Position>
</Post> 

<Window
  x:Class="wpftest.MainWindow"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:wpftest="clr-namespace:wpftest"
  Height="700"
  Width="700">


  <Window.Resources>
    <XmlDataProvider x:Key="XmlData" XPath="/root">
      <x:XData>
        <root xmlns="">
          <Post Id="0" Caption="aaaaaa" Code="US">
            <Position Caption="Строка 1" SysID="1" Number="5">
              <Post>
                <Mz Name="A1" Id="11" Value="548231"/>
                <Mt Name="Б2" Id="0" Value="42323"/>
                <Mt Name="Б3" Id="1" Value="31231"/>
              </Post>
            </Position>
          </Post>
          <Post Id="1" Caption="bbbbbb" Code="US">
            <Position Caption="Строка 1" SysID="1" Number="5">
              <Post>
                <Mz Name="A1" Id="11" Value="548231"/>
                <Mt Name="Б2" Id="0" Value="42323"/>
                <Mt Name="Б3" Id="1" Value="31231"/>
              </Post>
            </Position>
          </Post>
        </root>
      </x:XData>
    </XmlDataProvider>
    <HierarchicalDataTemplate x:Key="XmlTemplate">
      <HierarchicalDataTemplate.Resources>
        <wpftest:XmlElementConverter x:Key="XmlElementConverter" />
      </HierarchicalDataTemplate.Resources>
      <TextBlock Text="{Binding Converter={StaticResource XmlElementConverter}}" />
      <HierarchicalDataTemplate.ItemsSource>
        <Binding XPath="child::node()" />
      </HierarchicalDataTemplate.ItemsSource>
    </HierarchicalDataTemplate>
  </Window.Resources>

  <TreeView
    Margin="10"
    ItemsSource="{Binding Source={StaticResource XmlData}, XPath=Post}"
    ItemTemplate="{StaticResource XmlTemplate}" />
</Window>

namespace wpftest
{
  public partial class MainWindow
  {
    public MainWindow()
    {
      InitializeComponent();
    }
  }

  public class XmlElementConverter: IValueConverter
  {
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
      if (!(value is XmlElement xe)) return DependencyProperty.UnsetValue;
      var sb = new StringBuilder();
      sb.Append(xe.Name);
      foreach (XmlAttribute xa in xe.Attributes)
        sb.Append(" ").Append(xa.Name).Append("=\"").Append(xa.Value).Append("\"");
      return sb.ToString();
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
      throw new NotImplementedException();
    }
  }
}

public static class ICollectionExt
    {
        public static IEnumerable<T> ToIEnumerable<T>(this ICollection col) where T : class
        {
            if (col == null) return null;
            var ret = new List<T>();
            if (col.Count > 0)
            {
                var enu = col.GetEnumerator();
                enu.MoveNext();
                for (var i = 0; i < col.Count; i++)
                {
                    ret.Add(enu.Current as T);
                    enu.MoveNext();
                }
            }
            return ret;
        }
    }

return $"{xe.Name}{string.Join("", xe.Attributes.ToIEnumerable<XmlAttribute>().Select(xa => $" {xa.Name}=\"{xa.Value}\""))}";