本文告诉大家在 ListBox 做选择时，多选时 SelectedItem 和 SelectedIndex 的值。

首先写一个界面，两个按钮和一个ListBox ，点击第一个按钮选择多个元素，点击第二个按钮就显示当前的 SelectedItem 和 SelectedIndex

    <Grid>
        <ListBox x:Name="QjdckjpVemnepybg" SelectionMode="Multiple"></ListBox>
        <Button Width="100" Height="100" Click="ButtonBase_OnClick"></Button>
        <Button Margin="100,10,10,10" Width="100" Height="100" Click="RyltnqiUmqgwuz_OnClick"></Button>
    </Grid>

在点击前需要给 ListBox 数据，请看下面

        public MainWindow()
            InitializeComponent();
            QjdckjpVemnepybg.ItemsSource = FyuzbqklBxdk;
            for (int i = 0; i < 100; i++)
                FyuzbqklBxdk.Add(i.ToString());
        public ObservableCollection<string> FyuzbqklBxdk { get; } = new ObservableCollection<string>();

点击第一个按钮选择多个元素

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
            QjdckjpVemnepybg.SelectedItems.Clear();
            QjdckjpVemnepybg.SelectedItems.Add(FyuzbqklBxdk[2]);
            QjdckjpVemnepybg.SelectedItems.Add(FyuzbqklBxdk[5]);
        }

点击第二个按钮显示 SelectedItem 和 SelectedIndex

        private void RyltnqiUmqgwuz_OnClick(object sender, RoutedEventArgs e)
            Console.WriteLine(QjdckjpVemnepybg.SelectedItem);
            Console.WriteLine(QjdckjpVemnepybg.SelectedIndex);
        }

可以看到，点击第一个按钮之后，SelectedItem 和 SelectedIndex 都是 2 因为他会显示多选的第一个。

如果选择的是 SelectedItem ，那么 SelectedItems 就只有 SelectedItem 的元素

      private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
            QjdckjpVemnepybg.SelectedItem = FyuzbqklBxdk[2];
        private void RyltnqiUmqgwuz_OnClick(object sender, RoutedEventArgs e)
            Console.WriteLine(QjdckjpVemnepybg.SelectedItems.Count);
            foreach (var temp in QjdckjpVemnepybg.SelectedItems)
                Console.WriteLine(temp);
        }

如果选择是 SelectedIndex ，那么 SelectedItems 有对应序号的元素

    private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
            QjdckjpVemnepybg.SelectedIndex = 2;
        private void RyltnqiUmqgwuz_OnClick(object sender, RoutedEventArgs e)
            Console.WriteLine(QjdckjpVemnepybg.SelectedItems.Count);
            foreach (var temp in QjdckjpVemnepybg.SelectedItems)
                Console.WriteLine(temp);
        }

如果这时 SelectedIndex 为 -1 ，那么 SelectedItems 没有选择元素

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
            QjdckjpVemnepybg.SelectedIndex = -1;
        private void RyltnqiUmqgwuz_OnClick(object sender, RoutedEventArgs e)