Ensure there is at least one item selected


   
  
        
<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="WPF" Height="300" Width="300">
    <StackPanel>
        <ListBox FontSize="16" Height="150" Margin="5" Name="listBox1" SelectionMode="Extended">
            <ListBoxItem>List Item 1</ListBoxItem>
            <ListBoxItem>List Item 2</ListBoxItem>
            <ListBoxItem>List Item 3</ListBoxItem>
        </ListBox>
        <StackPanel HorizontalAlignment="Center" Orientation="Horizontal">
            <Label Content="_New item text:" VerticalAlignment="Center" Target="{Binding ElementName=textBox}"  />
            <TextBox Margin="5" Name="textBox" MinWidth="120" />            
        </StackPanel>
        <StackPanel HorizontalAlignment="Center" Orientation="Horizontal">
            <Button Click="btnAddListItem_Click" Content="Add Item" IsDefault="True" Margin="5" Name="btnAddListItem" />
            <Button Click="btnDeleteListItem_Click" Content="Delete Items" Margin="5" Name="btnDeleteListItem" />
            <Button Click="btnSelectAll_Click" Content="Select All" Margin="5" Name="btnSelectAll" />
        </StackPanel>
    </StackPanel>
</Window>
//File:Window.xaml.cs
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace WpfApplication1
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }
        private void btnAddListItem_Click(object sender, RoutedEventArgs e)
        {
            if (textBox.Text.Length == 0)
            {
                MessageBox.Show("Enter text to add to the list.", Title);
            }
            else
            {
                ListBoxItem item = new ListBoxItem();
                item.Content = textBox.Text;
                item.IsSelected = true;
                item.HorizontalAlignment = HorizontalAlignment.Center;
                item.FontWeight = FontWeights.Bold;
                item.FontFamily = new FontFamily("Tahoma");

                listBox1.Items.Add(item);

                textBox.Clear();
                textBox.Focus();
            }
        }

        private void btnDeleteListItem_Click(object sender, RoutedEventArgs e)
        {
            if (listBox1.SelectedItems.Count == 0)
            {
                MessageBox.Show("Select list items to delete.", Title);
            }
            else
            {
                while (listBox1.SelectedItems.Count > 0)
                {
                    listBox1.Items.Remove(listBox1.SelectedItems[0]);
                }
            }
        }
        private void btnSelectAll_Click(object sender, RoutedEventArgs e)
        {
            listBox1.SelectAll();
        }
    }
}