<Grid>

        <Grid.RowDefinitions>

            <RowDefinition Height="auto"></RowDefinition>

            <RowDefinition Height="*"></RowDefinition>

            <RowDefinition Height="auto"></RowDefinition>

        </Grid.RowDefinitions>

        <DockPanel Grid.Row="0">

            <TextBlock>Type here:</TextBlock>

            <TextBox Focusable="True" Width="710" PreviewKeyDown="PKeyUpAndDown" PreviewKeyUp="PKeyUpAndDown" KeyDown="PKeyUpAndDown" KeyUp="PKeyUpAndDown" PreviewTextInput="PTextInput" TextInput="PTextInput"></TextBox>

        </DockPanel>

        <ListBox Grid.Row="1" Margin="10,10,10,44" Name="mylistbox"></ListBox>

        <Button Click="Button_Click_2" Grid.Row="2" HorizontalAlignment="Right" Width="60" Margin="0,0,20,30">clear</Button>

    </Grid>

private void Button_Click_2(object sender, RoutedEventArgs e)

        {

            this.mylistbox.Items.Clear();

        }

        private void PKeyUpAndDown(object sender, KeyEventArgs e)

        {

            string message = "Event:" + e.RoutedEvent + "  " + "key:" + e.Key;

            this.mylistbox.Items.Add(message);

        }

        private void PTextInput(object sender, TextCompositionEventArgs e)

        {

            string message = "Event:" + e.RoutedEvent + "  " + "key:" + e.Text;

            this.mylistbox.Items.Add(message);

        }

    <Grid>

        <Grid.RowDefinitions>

            <RowDefinition></RowDefinition>

            <RowDefinition></RowDefinition>

        </Grid.RowDefinitions>

        <TextBox Grid.Row="0" Name="textbox1" TextWrapping="Wrap" SelectionChanged="TextBox_SelectionChanged">

            hello world

            hello world

            hello world

            hello world

        </TextBox>

        <TextBlock  Grid.Row="1" Name="tb"></TextBlock>

    </Grid>

        private void TextBox_SelectionChanged(object sender, RoutedEventArgs e)

        {

            if (this.textbox1.SelectionLength == 0) return;

            this.tb.Text = $"选择 {this.textbox1.SelectionStart} to {this.textbox1.SelectionLength} :{this.textbox1.SelectedText}";

        }