support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Controls/Base/SelectableItem.cs

@@ -0,0 +1,67 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Input;
+using System.Windows.Markup;
+using HandyControl.Data;
+
+namespace HandyControl.Controls
+{
+    [ContentProperty("Content")]
+    public class SelectableItem : Control, ISelectable
+    {
+        private bool _isMouseLeftButtonDown;
+
+        protected override void OnMouseLeave(MouseEventArgs e)
+        {
+            base.OnMouseLeave(e);
+
+            _isMouseLeftButtonDown = false;
+        }
+
+        protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
+        {
+            base.OnMouseLeftButtonDown(e);
+
+            _isMouseLeftButtonDown = true;
+        }
+
+        protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
+        {
+            base.OnMouseLeftButtonUp(e);
+
+            if (_isMouseLeftButtonDown)
+            {
+                RaiseEvent(new RoutedEventArgs(SelectedEvent, this));
+                _isMouseLeftButtonDown = false;
+            }
+        }
+
+        public static readonly DependencyProperty IsSelectedProperty = DependencyProperty.Register(
+            "IsSelected", typeof(bool), typeof(SelectableItem), new PropertyMetadata(ValueBoxes.FalseBox));
+
+        public bool IsSelected
+        {
+            get => (bool)GetValue(IsSelectedProperty);
+            set => SetValue(IsSelectedProperty, value);
+        }
+
+        public static readonly RoutedEvent SelectedEvent =
+            EventManager.RegisterRoutedEvent("Selected", RoutingStrategy.Bubble,
+                typeof(RoutedEventHandler), typeof(SelectableItem));
+
+        public event RoutedEventHandler Selected
+        {
+            add => AddHandler(SelectedEvent, value);
+            remove => RemoveHandler(SelectedEvent, value);
+        }
+
+        public static readonly DependencyProperty ContentProperty = DependencyProperty.Register(
+            "Content", typeof(object), typeof(SelectableItem), new PropertyMetadata(default(object)));
+
+        public object Content
+        {
+            get => GetValue(ContentProperty);
+            set => SetValue(ContentProperty, value);
+        }
+    }
+}