support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Controls/Attach/WindowAttach.cs

@@ -0,0 +1,41 @@
+using System.Windows;
+using System.Windows.Input;
+using HandyControl.Data;
+
+namespace HandyControl.Controls
+{
+    public static class WindowAttach
+    {
+        public static readonly DependencyProperty IsDragElementProperty = DependencyProperty.RegisterAttached(
+            "IsDragElement", typeof(bool), typeof(WindowAttach), new PropertyMetadata(ValueBoxes.FalseBox, OnIsDragElementChanged));
+
+        public static void SetIsDragElement(DependencyObject element, bool value)
+            => element.SetValue(IsDragElementProperty, value);
+
+        public static bool GetIsDragElement(DependencyObject element)
+            => (bool) element.GetValue(IsDragElementProperty);
+
+        private static void OnIsDragElementChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            if (d is UIElement ctl)
+            {
+                if ((bool)e.NewValue)
+                {
+                    ctl.MouseLeftButtonDown += DragElement_MouseLeftButtonDown;
+                }
+                else
+                {
+                    ctl.MouseLeftButtonDown -= DragElement_MouseLeftButtonDown;
+                }
+            }
+        }
+
+        private static void DragElement_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
+        {
+            if (sender is DependencyObject obj && e.ButtonState == MouseButtonState.Pressed)
+            {
+                System.Windows.Window.GetWindow(obj)?.DragMove();
+            }
+        }
+    }
+}