根据ListBox.SelectedIndex在WPF中启用按钮

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.ColumnDefinitions>
           <ColumnDefinition/>
           <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <ListBox x:Name="listBox" SelectionChanged="listBox_SelectionChanged">
            <ListBoxItem Content="Item 1"/>
            <ListBoxItem Content="Item 2"/>
            <ListBoxItem Content="Item 3"/>
        </ListBox>
       <Button x:Name="button" Content="Click me" IsEnabled="False" Grid.Column="1"/>
    </Grid>
</Window>

using System.Windows;

namespace WpfApp1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void listBox_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
        {
            if (listBox.SelectedIndex >= 0)
            {
                button.IsEnabled = true;
            }
            else
            {
                button.IsEnabled = false;
            }
        }
    }
}