support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Interactivity/AdornerContainer.cs

@@ -0,0 +1,40 @@
+using System.Windows;
+using System.Windows.Documents;
+using System.Windows.Media;
+
+namespace HandyControl.Interactivity
+{
+    public class AdornerContainer : Adorner
+    {
+        private UIElement _child;
+
+        public AdornerContainer(UIElement adornedElement) : base(adornedElement)
+        {
+        }
+
+        public UIElement Child
+        {
+            get =>
+                _child;
+            set
+            {
+                AddVisualChild(value);
+                _child = value;
+            }
+        }
+
+        protected override int VisualChildrenCount => _child != null ? 1 : 0;
+
+        protected override Size ArrangeOverride(Size finalSize)
+        {
+            _child?.Arrange(new Rect(finalSize));
+            return finalSize;
+        }
+
+        protected override Visual GetVisualChild(int index)
+        {
+            if (index == 0 && _child != null) return _child;
+            return base.GetVisualChild(index);
+        }
+    }
+}