Data at the root level is invalid. Line 1, position 1. XML is not valid
<p>I have two errors in my code: </p>

<ul>
<li>Data at the root level is invalid. Line 1, position 1.</li>
<li>Markup file is not valid. Specify a source markup file with an .xaml extension.   </li>
</ul>

<p>Why?</p>

<p>MainWindow.xaml:</p>

<pre><code>&lt;Window x:Class="TestWpfApplication.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="Eight Ball Answer" Height="328" Width="412" &gt;
    &lt;Grid Name="grid1"&gt;
        &lt;Grid.RowDefinitions&gt;
            &lt;RowDefinition Height="*" /&gt;
            &lt;RowDefinition Height="Auto" /&gt;
            &lt;RowDefinition Height="*" /&gt;
        &lt;/Grid.RowDefinitions&gt;
        &lt;TextBox VerticalAlignment="Stretch" HorizontalAlignment="Stretch"     Margin="10,10,13,10" Name="txtQuestion"      TextWrapping="Wrap" FontFamily="Verdana" FontSize="24"     Grid.Row="0"&gt;[Place question here.]&lt;/TextBox&gt;
        &lt;Button VerticalAlignment="Top" HorizontalAlignment="Left"     Margin="10,0,0,20" Width="127" Height="23" Name="cmdAnswer"     Click="cmdAnswer_Click" Grid.Row="1"&gt;Ask the Eight Ball&lt;/Button&gt;
        &lt;TextBox VerticalAlignment="Stretch" HorizontalAlignment="Stretch"     Margin="10,10,13,10" Name="txtAnswer" TextWrapping="Wrap"     IsReadOnly="True" FontFamily="Verdana" FontSize="24" Foreground="Green"     Grid.Row="2"&gt;[Answer will appear here.]&lt;/TextBox&gt;
        &lt;Grid.Background&gt;
            &lt;LinearGradientBrush&gt;
                &lt;LinearGradientBrush.GradientStops&gt;
                    &lt;GradientStop Offset="0.00" Color="Red" /&gt;
                    &lt;GradientStop Offset="0.50" Color="Indigo" /&gt;
                    &lt;GradientStop Offset="1.00" Color="Violet" /&gt;
                &lt;/LinearGradientBrush.GradientStops&gt;
            &lt;/LinearGradientBrush&gt;
        &lt;/Grid.Background&gt;
    &lt;/Grid&gt;
&lt;/Window&gt;
</code></pre>

<p>MainWindow.xaml.cs:</p>

<pre><code>using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace TestWpfApplication
{
    /// &lt;summary&gt;
    /// Interaction logic for MainWindow.xaml
    /// &lt;/summary&gt;
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void cmdAnswer_Click(object sender, RoutedEventArgs e)
        {
            this.Cursor = Cursors.Wait;            

            txtAnswer.Text = "AAA";
            this.Cursor = null;
        }
    }
}
</code></pre>

<p>Update 1:</p>

<ol>
<li>This page was created by copy&amp;paste method, not by designer.</li>
<li>MainWindow.xaml has property BuildAction set to Page.</li>
</ol>

<p>Is if helpful?</p>