<ComboBox x:Name="comboType" Grid.Column="1" Grid.Row="4" HorizontalAlignment="Left" VerticalAlignment="Center" Width="150" Text="{Binding Question.Type}">
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectionChanged">
<i:InvokeCommandAction Command="{Binding SetTypeCommand}" CommandParameter="{Binding SelectedIndex,ElementName=comboType}" />
</i:EventTrigger>
</i:Interaction.Triggers>
<ComboBoxItem Content="设置题型..."></ComboBoxItem>
<ComboBoxItem Content="单选题"></ComboBoxItem>
<ComboBoxItem Content="多选题"></ComboBoxItem>

</ComboBox>

private DelegateCommand<int> _setTypeCommand; 
public DelegateCommand<int> SetTypeCommand
{
get { return _setTypeCommand??(_setTypeCommand = new DelegateCommand<int>(SetType)); }
set { _setTypeCommand = value; }
}

private void SetType(int selectedIndex)
{
//
}