support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Interactivity/BehaviorCollection.cs

@@ -0,0 +1,42 @@
+using System.Windows;
+
+namespace HandyControl.Interactivity
+{
+    public sealed class BehaviorCollection : AttachableCollection<Behavior>
+    {
+        internal BehaviorCollection()
+        {
+        }
+
+        protected override void OnAttached()
+        {
+            foreach (var behavior in this)
+                behavior.Attach(AssociatedObject);
+        }
+
+        protected override void OnDetaching()
+        {
+            foreach (var behavior in this)
+                behavior.Detach();
+        }
+
+        internal override void ItemAdded(Behavior item)
+        {
+            if (AssociatedObject == null)
+                return;
+            item.Attach(AssociatedObject);
+        }
+
+        internal override void ItemRemoved(Behavior item)
+        {
+            if (((IAttachedObject) item).AssociatedObject == null)
+                return;
+            item.Detach();
+        }
+
+        protected override Freezable CreateInstanceCore()
+        {
+            return new BehaviorCollection();
+        }
+    }
+}