title: WPF Binding error
body: <p>I really don't understand what am I doing wrong here.
  These are my classes:</p>
  
  <pre><code>class Filtro
  {
      public string url;
      public string tag;
      public string tipo;
  
      public Filtro(string _tipo, string _tag, string _url)
      {
          url = _url;
          tag = _tag;
          tipo = _tipo;
      }
  
  }
  
  class MainWindow_ViewModel
  {
      private ObservableCollection&lt;Filtro&gt; _filtros;
      public ObservableCollection&lt;Filtro&gt; filtros
      {
          get
          {
              return _filtros;
          }
      }
  
      public MainWindow_ViewModel()
      {
          _filtros = new ObservableCollection&lt;Filtro&gt;();
          _filtros.Add(new Filtro("tipo1","tag1","url1"));
          _filtros.Add(new Filtro("tipo2","tag2","url2"));
          _filtros.Add(new Filtro("tipo3","tag3","url3"));
          _filtros.Add(new Filtro("tipo4","tag4","url4"));
          _filtros.Add(new Filtro("tipo5","tag5","url5"));
      }
  
  
  }
  
  public partial class MainWindow : Window
  {
      static public MainWindow wn;
      private MainWindow_ViewModel mwvm;
  
      public MainWindow()
      {
          wn = this;
          mwvm = new MainWindow_ViewModel();
      }
  }
  </code></pre>
  
  <p>I have a <code>ListBox</code> in XAML and I'm trying to bind it to <code>MainWindow.wn.mwvm.filtros</code>. (Let's say I want to bind it to the <code>Filtro.url</code> field).</p>
  
  <pre><code>&lt;Window x:Class="computrabajo.MainWindow"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:local="clr-namespace:computrabajo"
          Title="MainWindow" Height="350" Width="525"&gt;
      &lt;Window.DataContext&gt;
          &lt;local:MainWindow_ViewModel&gt;&lt;/local:MainWindow_ViewModel&gt;
      &lt;/Window.DataContext&gt;
  ...
  &lt;ListBox MinHeight="100" ItemsSource="{Binding filtros}"&gt;
  </code></pre>
  
  <p>With this code I get:</p>
  
  <blockquote>
    <p>Cannot create default converter to perform 'one-way' conversions
    between types
     'System.Collections.ObjectModel.ObservableCollection!1[computrabajo.Filtro]'
    and 'System.Windows.Data.BindingGroup'. Consider using Converter
    property of Binding. BindingExpression:Path=filtros;
    DataItem='MainWindow_ViewModel' (HashCode=38845522); target element is
    'ListBox' (Name=''); target property is 'ItemBindingGroup' (type
    'BindingGroup')</p>
  </blockquote>
  
  <p>PS: Changed to <code>ItemsSource</code> but now I get:</p>
  
  <blockquote>
    <p>Cannot retrieve value using the binding and no valid fallback value
    exists; using default instead.</p>
  </blockquote>