如何在ComboxItem中将DockPanel的子宽度设置为100％？

Question

<ComboBoxItem HorizontalContentAlignment="Stretch" Width="Auto">
     <DockPanel Background="Red" HorizontalAlignment="Stretch" LastChildFill="True" Width="Auto">
        <Label DockPanel.Dock="Left" Name="lbName" ></Label>
        <Image DockPanel.Dock="Right" HorizontalAlignment="Right" Name="image" Source="/Test;component/Images/cross.jpg" Width="16" Height="16" Stretch="UniformToFill" />
        <Image DockPanel.Dock="Right" HorizontalAlignment="Right" Name="image2" Source="/Test;component/Images/cross.jpg" Width="16" Height="16" Stretch="UniformToFill" />
    </DockPanel>
</ComboBoxItem>

如下图DockPanel（标记为红色）所示，ComboboxItem的宽度不是100％。

如何在XAML中将DockPanel拉伸到ComboboxItem的大小？

Answer 1

不要将DockPanel用于此类布局。改为使用Grid：

<ComboBoxItem HorizontalContentAlignment="Stretch" Width="Auto">
    <Grid>
        <Grid.ColumnDefinitions>
           <ColumnDefinition Width="*" />
           <ColumnDefinition Width="Auto" />
           <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        <Label Grid.Column="0" Name="lbName" ></Label>
        <Image Grid.Column="1" Name="image" Source="/Test;component/Images/cross.jpg" Width="16" Height="16" Stretch="UniformToFill" />
        <Image Grid.Column="2" Name="image2" Source="/Test;component/Images/cross.jpg" Width="16" Height="16" Stretch="UniformToFill" />
    </Grid>
</ComboBoxItem>

Answer 2

事实证明，只有当事件SelectionChanged被触发时，ComboBoxItem的内容才会填满整个空间。

示例：

<强> XAML

<ComboBox Width="300"
          Height="30"
          SelectionChanged="ComboBox_SelectionChanged">

    <ComboBoxItem>Test</ComboBoxItem>

    <ComboBoxItem Name="comboBoxItem"
                  HorizontalContentAlignment="Stretch"                          
                  Width="Auto">

        <DockPanel Background="Red" 
                   HorizontalAlignment="Stretch" 
                   Width="Auto">                    
            ...
        </DockPanel>
    </ComboBoxItem>
</ComboBox>

<强> Code-behind

private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    MessageBox.Show(comboBoxItem.ActualWidth.ToString());
}

当您启动ComboBoxItem的应用程序ActualWidth为零时，但当SelectionChanged事件触发的值为298时。

<强> Workaround

要解决方法，请在开头添加ComboBoxItem，例如：Select item并设置为ComboBox SelectedIndex="0"，如下所示：

<ComboBox Width="300"
          Height="30"
          SelectedIndex="0">

    <ComboBoxItem>Select item</ComboBoxItem>
    ...