Binding of the control created by xaml works fine, but binding of the control created by code does not work

// MainWindow.xaml.cs

namespace TestApp2
{
    public sealed partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

<!--MainWinodw.xaml-->

<Window x:Class="TestApp2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ex="using:TestApp2">
    <!--<ex:CustomButton/>-->
</Window>

// App.xaml.cs

namespace TestApp2
{
    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();
        }

        protected override void OnLaunched(LaunchActivatedEventArgs args)
        {
            var mainWindow = new MainWindow();
            mainWindow.Content = new CustomButton();
            mainWindow.Activate();
        }
    }
}

// CustomButton.cs

namespace TestApp2
{
    internal class CustomButton : Button
    {
        public object MyProperty
        {
            get => GetValue(MyPropertyProperty);
            set => SetValue(MyPropertyProperty, value);
        }
        public static readonly DependencyProperty MyPropertyProperty =
            DependencyProperty.Register(nameof(MyProperty), typeof(object), typeof(CustomButton), new(null));

        public CustomButton()
        {
            SetBinding(ContentProperty, new Binding
            {
                Source = this,
                Path = new(nameof(MyProperty)),
            });

            // This code should always update the binding,
            // but it does not work if you create a control in the code.
            MyProperty = "Something";
        }
    }
}

When you run the above code, only the null value, which is the value at the first binding, is loaded and not updated after that.

However, if you comment mainWindow.Content = new CustomButton(); out and uncomment <!--<ex:CustomButton/>--> to create it in xaml instead of in the code, the binding will work fine.

I tried setting the binding in the Loaded event because I thought it might be related to initialization, but the result was the same.

Why does the binding fail?

I'm trying to create inheritable pages or custom controls, so I'm trying to create controls and bindings from code without using xaml.

It seems that the binding breaks when you create the control programmatically.

Instead you can use the property changed event:

internal class CustomButton : Button
{
    public static readonly DependencyProperty MyPropertyProperty =
        DependencyProperty.Register(
            nameof(MyProperty),
            typeof(object),
            typeof(CustomButton),
            new PropertyMetadata(null, OnMyPropertyPropertyChanged));

    private static void OnMyPropertyPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is CustomButton customButton)
        {
            customButton.Content = e.NewValue;
        }
    }

    public object MyProperty
    {
        get => GetValue(MyPropertyProperty);
        set => SetValue(MyPropertyProperty, value);
    }
}