Fix #1531 by making IStyle an IAvaloniaObject.

src/Avalonia.Controls/Design.cs

@@ -1,5 +1,6 @@
 
 using System.Runtime.CompilerServices;
+using Avalonia.Styling;
 
 namespace Avalonia.Controls
 {
@@ -45,23 +46,18 @@ namespace Avalonia.Controls
         {
             return control.GetValue(DataContextProperty);
         }
-
-        static readonly ConditionalWeakTable<object, Control> Substitutes = new ConditionalWeakTable<object, Control>();
-
+        
         public static readonly AttachedProperty<Control> PreviewWithProperty = AvaloniaProperty
-            .RegisterAttached<AvaloniaObject, Control>("PreviewWith", typeof (Design));
+            .RegisterAttached<IStyle, Control>("PreviewWith", typeof (Design));
 
-        public static void SetPreviewWith(object target, Control control)
+        public static void SetPreviewWith(IStyle target, Control control)
         {
-            Substitutes.Remove(target);
-            Substitutes.Add(target, control);
+            target.SetValue(PreviewWithProperty, control);
         }
 
-        public static Control GetPreviewWith(object target)
+        public static Control GetPreviewWith(IStyle target)
         {
-            Control rv;
-            Substitutes.TryGetValue(target, out rv);
-            return rv;
+            return target.GetValue(PreviewWithProperty);
         }
 
         public static void ApplyDesignModeProperties(Control target, Control source)

src/Avalonia.Styling/Styling/IStyle.cs

@@ -8,7 +8,7 @@ namespace Avalonia.Styling
     /// <summary>
     /// Defines the interface for styles.
     /// </summary>
-    public interface IStyle : IResourceNode
+    public interface IStyle : IAvaloniaObject, IResourceNode
     {
         /// <summary>
         /// Attaches the style to a control if the style's selector matches.

src/Avalonia.Styling/Styling/Style.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Styling
     /// <summary>
     /// Defines a style.
     /// </summary>
-    public class Style : IStyle, ISetStyleParent
+    public class Style : AvaloniaObject, IStyle, ISetStyleParent
     {
         private static Dictionary<IStyleable, List<IDisposable>> _applied =
             new Dictionary<IStyleable, List<IDisposable>>();