support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Controls/Attach/VisualElement.cs

@@ -0,0 +1,17 @@
+using System.Windows;
+using System.Windows.Media;
+
+namespace HandyControl.Controls
+{
+    public class VisualElement
+    {
+        public static readonly DependencyProperty HighlightBrushProperty = DependencyProperty.RegisterAttached(
+            "HighlightBrush", typeof(Brush), typeof(VisualElement), new PropertyMetadata(default(Brush)));
+
+        public static void SetHighlightBrush(DependencyObject element, Brush value)
+            => element.SetValue(HighlightBrushProperty, value);
+
+        public static Brush GetHighlightBrush(DependencyObject element)
+            => (Brush) element.GetValue(HighlightBrushProperty);
+    }
+}