Why is my ContentDialog in WinUI 3 displaying empty even though my XAML has a TextBox in it?

WinUI 3 with C#:

I have my app's main Window class that displays a simple dialog when a menu is clicked:

private async void MyMenu_Click(object sender, RoutedEventArgs e)
{
    ContentDialog dialog = new ContentDialog()
    {
        XamlRoot = this.Content.XamlRoot,
        Title = "My Dialog",
        Content = new MyContentDialog(),
        PrimaryButtonText = "OK",
        CloseButtonText = "Cancel"
    };
    ContentDialogResult result = await dialog.ShowAsync();
}

This is the code-behind for the MyContentDialog class:

namespace myapp
{
    public sealed partial class MyContentDialog : ContentDialog
    {
        public MyContentDialog()
        {
            this.InitializeComponent();
        }
    }
}

And here is the XAML for the MyContentDialog class:

<ContentDialog
    x:Class="myapp.MyContentDialog"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:myapp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">

    <Grid>
        <TextBox x:Name="MyTextBox" />
    </Grid>
</ContentDialog>

Seems pretty basic, right? So why is my dialog appearing like this with no TextBox in it? It doesn't matter what UI controls I add to the XAML, I can't get anything to appear. Why?

You are already setting the Content here:

<ContentDialog
    x:Class="myapp.MyContentDialog"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:myapp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Style="{StaticResource DefaultContentDialogStyle}">

    <!-- Content -->
    <Grid>
        <TextBox x:Name="MyTextBox" />
    </Grid>

</ContentDialog>

So, you are overriding the Content here. You just need to remove the code setting the Content.

MyContentDialog dialog = new()
{
    XamlRoot = this.Content.XamlRoot,
    Title = "My Dialog",
    //Content = new MyContentDialog(), 
    PrimaryButtonText = "OK",
    CloseButtonText = "Cancel"
};

User Andrew KeepCoding led me in the right direction with his partial answer, but here is the fully corrected code.

I needed to call the MyContentDialog class as a dialog directly, remove the Content = new MyContentDialog() line, and add a line to update the style to look like the default style.

MyContentDialog dialog = new MyContentDialog()
{
    XamlRoot = this.Content.XamlRoot,
    Style = Application.Current.Resources["DefaultContentDialogStyle"] as Style,
    Title = "My Dialog",
    PrimaryButtonText = "OK",
    CloseButtonText = "Cancel"
};
ContentDialogResult result = await dialog.ShowAsync();