有没有办法在 MahApps.Metro Flyout 中设置 ComboBox 的 SelectedIndex？

Question

I have a UserControl that is contained in a MahApps.Metro.Controls.Flyout . Inside that UserControl , there is a ComboBox . I'm trying to set the default SelectedIndex to 0. So that when the flyout is opened, the first item ("Source A") is selected.

Currently, whenever the flyout is opened, the SelectedIndex is being set to -1 and nothing is selected.

View that holds the Flyout:

<mah:FlyoutsControl>
    <mah:Flyout IsOpen="{Binding MyViewIsOpen, Mode=TwoWay, 
    UpdateSourceTrigger=PropertyChanged}"
    x:Name="DataSourceFlyout">
        <Grid>
            <local:DataSourceView/>
        </Grid>
    </mah:Flyout>
</mah:FlyoutsControl>

DataSourceView:

<UserControl ...>
    <ComboBox SelectedIndex="0" 
      SelectedValue="{Binding SelectedDataSource, Mode=TwoWay, 
      UpdateSourceTrigger=PropertyChanged}"
      SynchronizedWithCurrentItem="True">
        <ComboBoxItem Tag="SourceA" Content="Source A"/>
        <ComboBoxItem Tag="SourceB" Content="Source B"/>
        <ComboBoxItem Tag="SourceC" Content="Source C"/>
    </ComboBox>
</UserControl>

ViewModel:

public string SelectedDataSource { get; set; }

Note: This seems to be caused by my UserControl being within a Flyout . The same XAML works fine when used outside of a Flyout . Is there something else I need to do to get the first item selected in my ComboBox ?

Answer 1

The source property in the view model determines the value to be selected.

It's unclear what the value of the SelectedDataSource property is in your example but you should set it to either "SourceA", "SourceB" or "SourceC" and then set the SelectedValuePath property to "Tag":

<ComboBox SelectedValue="{Binding SelectedDataSource, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
          SelectedValuePath="Tag">
    <ComboBoxItem Tag="SourceA" Content="Source A"/>
    <ComboBoxItem Tag="SourceB" Content="Source B"/>
    <ComboBoxItem Tag="SourceC" Content="Source C"/>
</ComboBox>

View Model:

public string SelectedDataSource { get; set; } = "SourceA";

If you intend to specify the selected value in the view using the SelectedIndex property, you should remove the binding.