wpf ComboBox设置默认值

最新的wpf的ComboBox设置默认值得方法是，给VM中的数据集合第一个元素插入一个提示项目，比如：请选择一项，然后通过数据绑定可以实现默认选中第一项，下面我就贴一下示例代码：
xaml页面:

<ComboBox Name="ResizeCanvas" SelectionChanged="ResizeCanvas_SelectionChanged" SelectedItem="{Binding Path=SelectedCanvasSize}" ItemsSource="{Binding Path=CnavasSizeList}" DisplayMemberPath="RatioText" SelectedValuePath="RatioValue"/>

 
相应的VM里面的数据源处理：这里页面不仅绑定了数据源，也绑定了ComboBox选中项，只要选中更改事件ResizeCanvas_SelectionChanged中去判断一下是不是为第一项就可以了，当选中第一项的时候不做处理
public MainWindowViewModel()
       {
           CanvasSize canvasSize0 = new CanvasSize("请选中一项", 0.0);
           CanvasSize canvasSize1 = new CanvasSize("100%", 1.0);
           CanvasSize canvasSize2 = new CanvasSize("125%", 1.25);
           CanvasSize canvasSize3 = new CanvasSize("150%", 1.5);
           CnavasSizeList.Add(canvasSize1);
           CnavasSizeList.Add(canvasSize2);
           CnavasSizeList.Add(canvasSize3);
selectedCanvasSize = canvasSize0;
       }
       private ObservableCollection<CanvasSize> canvasSizeList = new ObservableCollection<CanvasSize>();
       public ObservableCollection<CanvasSize> CnavasSizeList
       {
           get { return canvasSizeList; }
           set { canvasSizeList = value; OnPropertyChanged("CnavasSizeList"); }
       }
       private CanvasSize selectedCanvasSize;
       public CanvasSize SelectedCanvasSize
       {
           get { return selectedCanvasSize; }
           set { selectedCanvasSize = value; OnPropertyChanged("SelectedCanvasSize"); }

老方式，感觉不是很好用哈

在wpf里面简单的设置 ComboBox的text属性是不能设置它的默认选中的值的。在网上看到有人说可以设置默认选中项，设置它的selectedIndex="0".把第一项作为它的默认项。

在展开下拉框时，将第一项的visibility="collapsed"这样就行了。

<ComboBox Width="100" SelectedIndex="0">

<ComboBoxItem Content="Import" Visibility="Collapsed"></ComboBoxItem>

<ComboBoxItem Content="Import File" ></ComboBoxItem>

<ComboBoxItem Content="Import Folder"></ComboBoxItem>

</ComboBox>