xaml类中缺少Run方法

public class MyTextBlock : TextBlock
{
    // ...
}

public class MyTextBlock : TextBlock
{
    public void Run()
    {
        this.Text = "Hello, World!";
    }
}

<Window x:Class="MyApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:MyApp"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <local:MyTextBlock x:Name="myTextBlock" />
    </Grid>
</Window>

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