From my testing, it seems that the DataContext seems to be always assigned first before the assignment of Dependency Property, if I only set the DataContext and the Dependency Property once via XAML ( see the example below).

But is this always true? Is there a C# language spec that says– no, guarantees — this? I need this because my program depends precisely on this behavior.

Here’s the code that I can use to illustrate this point:

View Models

public class MainWindowVM
{
    public SimpleTextBoxVM SimpleText => new SimpleTextBoxVM();
}
public class SimpleTextBoxVM
{
}

public class Updater
{
}

UI

public class Updater
{
}

public partial class SimpleTextBoxExt : UserControl
{
    public SimpleTextBoxExt()
    {
        InitializeComponent();
        DataContextChanged += OnDataContextChanged;
    }

    private void OnDataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue != null)
        {
            Console.WriteLine("DataContext Assigned."); //this seems always executed first

        }
    }

    public static readonly DependencyProperty FractionalNumberProperty = DependencyProperty.Register(
        nameof(FractionalNumber), typeof(Updater), typeof(SimpleTextBoxExt), new PropertyMetadata(default(Updater), PropertyChangedCallback));

    private static void PropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is SimpleTextBoxExt simpleTextBox)
        {
            Console.WriteLine("Dependency property set"); //this seems always executed later
        }
    }



    public Updater FractionalNumber
    {
        get { return (Updater)GetValue(FractionalNumberProperty); }
        set { SetValue(FractionalNumberProperty, value); }
    }
}

<Window x:Class="DependencyPropertiesUI.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:DependencyPropertiesUI"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
   <Window.DataContext>
       <local:MainWindowVM/>
   </Window.DataContext>
    <local:SimpleTextBoxExt DataContext="{Binding SimpleText}">
        <local:SimpleTextBoxExt.FractionalNumber>
            <local:Updater/>
        </local:SimpleTextBoxExt.FractionalNumber>
    </local:SimpleTextBoxExt>
</Window>