support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Data/ValueBoxes.cs

@@ -0,0 +1,36 @@
+namespace HandyControl.Data
+{
+    /// <summary>
+    ///     装箱后的值类型（用于提高效率）
+    /// </summary>
+    internal static class ValueBoxes
+    {
+        internal static object TrueBox = true;
+
+        internal static object FalseBox = false;
+
+        internal static object Double0Box = .0;
+
+        internal static object Double1Box = 1.0;
+
+        internal static object Double20Box = 20.0;
+
+        internal static object Double100Box = 100.0;
+
+        internal static object Double200Box = 200.0;
+
+        internal static object Double300Box = 300.0;
+
+        internal static object DoubleNeg1Box = -1.0;
+
+        internal static object Int0Box = 0;
+
+        internal static object Int1Box = 1;
+
+        internal static object Int2Box = 2;
+
+        internal static object Int5Box = 5;
+
+        internal static object BooleanBox(bool value) => value ? TrueBox : FalseBox;
+    }
+}