While WPF and XAML make the common 90% of UI programming quite easy, sometimes it gets a little odd in those other 10%. For instance - the visual tree. Most of the time it works great, and you never need to do anything special with it. But what about when you specifically want to give a user control children? Or maybe you want to give an adorner an actual visual child, instead of using OnRender? It isn‘t really obvious right away how to go about doing that.

But while it is not obvious, it is possible - and so today we are going to take a look at using VisualCollection to do those things. Specifically, we are going to create an Adorner to be used in an AdornerLayer that accepts a Visual as content. This is not possible right out of the gate, because Adorners don‘t have any built in way to have Visual children, and we can‘t use any of the normal controls that do (Panel, ContentPresenter, etc...), because only Adorners can be placed on an AdornerLayer.

Now, the example we are going to build today to use this adorner that can present content is pretty silly, but it is actually quite useful. I‘ve used it a number of times for drag and drop (giving visual representations of what is being dragged and what will happen when it drops). Ok, now for some code:

public class AdornerContentPresenter : Adorner
{
  private VisualCollection _Visuals;
  private ContentPresenter _ContentPresenter;

  public AdornerContentPresenter(UIElement adornedElement)
    : base(adornedElement)
  {
    _Visuals = new VisualCollection(this);
    _ContentPresenter = new ContentPresenter();
    _Visuals.Add(_ContentPresenter);
  }

  public AdornerContentPresenter(UIElement adornedElement, Visual content)
    : this(adornedElement)
  { Content = content; }

  protected override Size MeasureOverride(Size constraint)
  {
    _ContentPresenter.Measure(constraint);
    return _ContentPresenter.DesiredSize;
  }

  protected override Size ArrangeOverride(Size finalSize)
  {
    _ContentPresenter.Arrange(new Rect(0, 0, 
         finalSize.Width, finalSize.Height));
    return _ContentPresenter.RenderSize;
  }

  protected override Visual GetVisualChild(int index)
  { return _Visuals[index]; }

  protected override int VisualChildrenCount
  { get { return _Visuals.Count; } }

  public object Content
  {
    get { return _ContentPresenter.Content; }
    set {