c# WPF TextBox 按索引检索行时的错误?

这里有简单的 wpf 应用程序:

Simple wpf app here:

MainWindow.xaml:

MainWindow.xaml:

<Window x:Class="wpf_textbox_of.MainWindow"
        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:wpf_textbox_of"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="20"/>
            <RowDefinition Height="20"/>
        </Grid.RowDefinitions>
        <TextBox Grid.Row="0" Margin="5"  Name="txt" AcceptsReturn="True" AcceptsTab="True" />
        <Button Grid.Row="1" Content="Fire 1!" Click="btn_OnClick" />
        <Button Grid.Row="2" Content="Fire 2!" Click="btn2_OnClick" />
    </Grid>
</Window>

MainWindow.cs(省略使用)

MainWindow.cs (omitting using)

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

    private void btn_OnClick(object sender, RoutedEventArgs e)
    {
        var sb = new StringBuilder();
        for (int i = 0; i < 90009; i++)
            sb.AppendLine($"{i}");
        txt.Text = sb.ToString();
    }

    private void btn2_OnClick(object sender, RoutedEventArgs e)
    {
        var sb = new StringBuilder();
        for (var i = 1; i < 7; i++)
            sb.AppendLine(txt.GetLineText(150 * i).Trim());
        for (var i = 1; i < 7; i++)
            sb.AppendLine(txt.GetLineText(15000*i).Trim());
        txt.Text = sb.ToString();
    }
}

跑.单击开火 1！"，然后单击开火 2！".

Run. Click "Fire 1!", then "Fire 2!".

预期结果:

150 300 450 600 750 900 15000 30000 45000 60000 75000 90000

150 300 450 600 750 900 15000 30000 45000 60000 75000 90000

但获得:

150 300 450 600 750 900 15001 30002 45003 60004 75004 90005

150 300 450 600 750 900 15001 30002 45003 60004 75004 90005

在 MSVS 2017、.net 4.6.1、.net 4.6、.net 4.5.2、.net 4.5、.net 4 上检查.有什么解决方法吗?

Checked on MSVS 2017, .net 4.6.1, .net 4.6, .net 4.5.2, .net 4.5, .net 4. Any workarounds?