Question

I have created window derived class (WindowAttachedCollection.MyWindow) and attached property which holds collection of these windows. But WPF designer in VS 2010 tries to create WindowInstance object for each window in that collection and it throws ArgumentException:

The value “Microsoft.Expression.Platform.WPF.InstanceBuilders.WindowInstance” is not of type “WindowAttachedCollection.MyWindow” and cannot be used in this generic collection.
Parameter name: value

So it breaks WPF designer.

Is there any way how to disable instancing WindowInstance instead of MyWindow in WPF designer? At this time I don’t require any design-time support for this collection of MyWindow.

EDIT:

public static readonly DependencyPropertyKey DialogsPropertyKey = DependencyProperty.RegisterAttachedReadOnly(
    "DialogsInternal",
    typeof(ObservableCollection<MyWindow>),
    typeof(MyWindow),
    new PropertyMetadata(null));

public static readonly DependencyProperty DialogsProperty = DialogsPropertyKey.DependencyProperty;

public static void SetDialogs(UIElement element, ObservableCollection<MyWindow> value)
{
    element.SetValue(DialogsPropertyKey, value);
}

public static ObservableCollection<MyWindow> GetDialogs(UIElement element)
{
    var dialogs = (ObservableCollection<MyWindow>)element.GetValue(DialogsProperty);
    if (dialogs == null)
    {
        dialogs = new ObservableCollection<MyWindow>();
        SetDialogs(element, dialogs);
    }

    return dialogs;
}

Answer 1

I decided to change base class of MyWindow from Window to ContentControl. For our purposes it is sufficient. Each ContentControl is wrapped into a Window when becomes active.