怎么让 Combobox 在展开的时候 让文本框里的内容继续显示选中项的内容而不是空白?

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• 2009年11月4日 2:50Allen Chen - MSFTMSFT, 版主

   
  

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  你好,
  由于内置的控件把代码写死了:
  private void OnIsDropDownOpenChanged(bool isDropDownOpen)
  {
      if (isDropDownOpen)
      {
          if (this.ElementPopup != null)
          {
              this.ElementPopup.IsOpen = true;
          }
          if (this.ElementDropDownToggle != null)
          {
              this.ElementDropDownToggle.IsChecked = true;
          }
          this.SetContentPresenter((base.SelectedItem is FrameworkElement) ? -1 : base.SelectedIndex);
          this.OnDropDownOpened(EventArgs.Empty);
      }
  ...
  可以看到当DropDownOpen的时候会调用this.SetContentPresenter((base.SelectedItem is FrameworkElement) ? -1 : base.SelectedIndex);
  大部分情况下SelectedItem 就是源自FrameworkElement所以等于调用了this.SetContentPresenter(-1).
  再看看SetContentPresenter方法:
  
  private void SetContentPresenter(int index)
  {
   ...      
     if (index == -1)
      {
          if (this.ElementContentPresenter != null)
          {
              this.ElementContentPresenter.Content = this._emptyContent;
              this.ElementContentPresenter.ContentTemplate = null;
          }
          this.SelectionBoxItem = null;
          this.SelectionBoxItemTemplate = null;
      }
  ...
  index为-1时会把ElementContentPresenter.Content 设置为_emptyContent. ElementContentPresenter为ComboBox模板内的ContentPresenter, _emptyContent则是ComboBox模板内ContentPresenter的最初的Content:
  
   <Border x:Name="ContentPresenterBorder">
                              <Grid>
                                  <ToggleButton x:Name="DropDownToggle" HorizontalAlignment="Stretch" Margin="0" Style="{StaticResource comboToggleStyle}" VerticalAlignment="Stretch" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" HorizontalContentAlignment="Right">
                                      <Path x:Name="BtnArrow" Stretch="Uniform" Height="4" HorizontalAlignment="Right" Margin="0,0,6,0" Width="8" Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z ">
                                          <Path.Fill>
                                              <SolidColorBrush x:Name="BtnArrowColor" Color="#FF333333"/>
                                          </Path.Fill>
                                      </Path>
                                  </ToggleButton>
                                  <ContentPresenter x:Name="ContentPresenter" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                                      <TextBlock/>
                                  </ContentPresenter>
                              </Grid>
                          </Border>
  
  解决办法就是在 this.SetContentPresenter调用之后,再次设置ContentPresenter的Content.我写了一个简单的控件,你可以参考下如何做到:
  
  http://cid-2fa13ebc6cc8e80f.skydrive.live.com/self.aspx/Public/MyComboBox.zip
  
  测试代码:
  
  

   

   

  <c:MyComboBox Width="200" Height="50">

   

   

  <ComboBoxItem Content="a"/>

   

   

  <ComboBoxItem Content="b"/>

   

   

  <ComboBoxItem Content="c"/>

   

   

  </c:MyComboBox>

  
  

  ---

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  • 已标记为答案zhang xiaolei 2009年11月4日 5:28
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• 2009年11月4日 5:28zhang xiaolei

   
  

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  辛苦了  多谢!     其实我就是想问 在不重新写控件的情况下 Combobox 本身能不能解决问题   看来是不能了. 
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