Removed sealed constraint from class declaration to allow inheritance.

Performs casting of extended component only once in OnInitialized, and casts to only necessary level (i.e. FrameworkElement) to allow inherited classes to support other panels that maps to Canvas placement properties (using DependencyProperty.AddOwner).

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.Designer/Project/Extensions/CanvasPlacementSupport.cs

@@ -30,9 +30,11 @@ namespace ICSharpCode.WpfDesign.Designer.Extensions
 	/// Provides <see cref="IPlacementBehavior"/> behavior for <see cref="Canvas"/>.
 	/// </summary>
 	[ExtensionFor(typeof(Canvas), OverrideExtension=typeof(DefaultPlacementBehavior))]
-	public sealed class CanvasPlacementSupport : SnaplinePlacementBehavior
+	public class CanvasPlacementSupport : SnaplinePlacementBehavior
 	{
 		GrayOutDesignerExceptActiveArea grayOut;
+		FrameworkElement extendedComponent;
+		FrameworkElement extendedView;
 		
 		static double GetCanvasProperty(UIElement element, DependencyProperty d)
 		{
@@ -48,6 +50,14 @@ namespace ICSharpCode.WpfDesign.Designer.Extensions
 			return element.ReadLocalValue(d) != DependencyProperty.UnsetValue;
 		}
 		
+		protected override void OnInitialized()
+		{
+			base.OnInitialized();
+
+			extendedComponent = (FrameworkElement)ExtendedItem.Component;
+			extendedView = (FrameworkElement)this.ExtendedItem.View;
+		}
+		
 		public override void SetPosition(PlacementInformation info)
 		{
 			base.SetPosition(info);
@@ -58,7 +68,7 @@ namespace ICSharpCode.WpfDesign.Designer.Extensions
 
 			if (IsPropertySet(child, Canvas.RightProperty))
 			{
-				var newR = ((Canvas) ExtendedItem.Component).ActualWidth - newPosition.Right;
+				var newR = extendedComponent.ActualWidth - newPosition.Right;
 				if (newR != GetCanvasProperty(child, Canvas.RightProperty))
 					info.Item.Properties.GetAttachedProperty(Canvas.RightProperty).SetValue(newR);
 			}
@@ -70,7 +80,7 @@ namespace ICSharpCode.WpfDesign.Designer.Extensions
 
 			if (IsPropertySet(child, Canvas.BottomProperty))
 			{
-				var newB = ((Canvas)ExtendedItem.Component).ActualHeight - newPosition.Bottom;
+				var newB = extendedComponent.ActualHeight - newPosition.Bottom;
 				if (newB != GetCanvasProperty(child, Canvas.BottomProperty))
 					info.Item.Properties.GetAttachedProperty(Canvas.BottomProperty).SetValue(newB);
 			}
@@ -81,8 +91,7 @@ namespace ICSharpCode.WpfDesign.Designer.Extensions
 
 			if (info.Item == Services.Selection.PrimarySelection)
 			{
-				var cv = this.ExtendedItem.View as Canvas;
-				var b = new Rect(0, 0, cv.ActualWidth, cv.ActualHeight);
+				var b = new Rect(0, 0, extendedView.ActualWidth, extendedView.ActualHeight);
 				// only for primary selection:
 				if (grayOut != null)
 				{