support for .NET framework 3.5

Still have bugs

Net35/HandyControlDemo_Net35/ViewModel/ComboBoxDemoViewModel.cs

@@ -0,0 +1,25 @@
+using System.Collections.Generic;
+using GalaSoft.MvvmLight;
+using HandyControlDemo.Service;
+
+namespace HandyControlDemo.ViewModel
+{
+    public class ComboBoxDemoViewModel : ViewModelBase
+    {
+        /// <summary>
+        ///     数据列表
+        /// </summary>
+        private List<string> _dataList;
+
+        /// <summary>
+        ///     数据列表
+        /// </summary>
+        public List<string> DataList
+        {
+            get => _dataList;
+            set => Set(nameof(DataList), ref _dataList, value);
+        }
+
+        public ComboBoxDemoViewModel(DataService dataService) => DataList = dataService.GetComboBoxDemoDataList();
+    }
+}