Usercontrol binding lost on value updated, that doesn't happen with standard textbox control

Question

I have a problem with binding in usercontrol.

This is my usercontrol:

UserControl1.xaml

<UserControl x:Class="WpfApp1.UserControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="http://schemas.openxmlformats.org/markup-ompatibility/2006" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
    xmlns:local="clr-namespace:WpfApp1"
    mc:Ignorable="d" 
    x:Name="usercontrol"
    d:DesignHeight="450" d:DesignWidth="800">
    <Grid>
        <TextBox Text="{Binding HmiField, ElementName=usercontrol}"/>
    </Grid>
</UserControl>

UserControl1.xaml.cs

namespace WpfApp1
{
    public partial class UserControl1 : UserControl
    {

        public double HmiField
        {
            get { return (double)GetValue(HmiFieldProperty); }
            set { SetValue(HmiFieldProperty, value); }
        }
        public static readonly DependencyProperty HmiFieldProperty =
            DependencyProperty.Register("HmiField", typeof(double), typeof(UserControl1));

        public UserControl1()
        {
            InitializeComponent();
        }
    }
}

And this is the main window:

MainWindow.xaml

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d"
        DataContext="{Binding Md, RelativeSource={RelativeSource Self}}"
        Title="MainWindow" Height="450" Width="800">
    <UniformGrid>
        <Button Content="{Binding Prop1}" Click="Button_Click"/>
        <Label Content="{Binding Prop1}"/>
        <TextBox Text="{Binding Prop1}"/>
        <local:UserControl1 HmiField="{Binding Prop1}"/>
    </UniformGrid>
</Window>

MainWindow.xaml.cs

namespace WpfApp1
{

    public class tMd: INotifyPropertyChanged
    {
        #region Interfaccia INotifyPropertyChanged

        public event PropertyChangedEventHandler PropertyChanged;

        public void NotifyPropertyChanged(string propName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
        }

        #endregion

        private double prop1;
        public double Prop1 { get
            {
                return prop1;
            }
            set
            {
                if (prop1 != value)
                {
                    prop1 = value;
                    NotifyPropertyChanged("Prop1");
                }
            }
        }
    }

    public partial class MainWindow : Window
    {

        public tMd Md
        {
            get { return (tMd)GetValue(MdProperty); }
            set { SetValue(MdProperty, value); }
        }
        public static readonly DependencyProperty MdProperty =
            DependencyProperty.Register("Md", typeof(tMd), typeof(MainWindow), new PropertyMetadata(new tMd()));

        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Md.Prop1 = 1234.5678;
        }
    }
}

I found some similar question:

How do I change TextBox.Text without losing the binding in WPF?

WPF: Binding is lost when bindings are updated

WPF Textbox TwoWay binding in datatemplate not updating the source even on LostFocus

But I can't completely understand what's happening: why a standard textbox work as expected and my usercontrol no?

Or better: is there a way to have my usercontrol works with the textbox's behaviour?

Answer 1

Your Prop1 notifies when it changes, but you haven't told you binding to trigger on that notification. Try including UpdateSourceTrigger=PropertyChanged in you binding

Answer 2

The Binding must be TwoWay, either set explicitly

<local:UserControl1 HmiField="{Binding Prop1, Mode=TwoWay}"/>

or implicitly by default:

public static readonly DependencyProperty HmiFieldProperty =
    DependencyProperty.Register(
        nameof(HmiField), typeof(double), typeof(UserControl1),
        new FrameworkPropertyMetadata(
            0d, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

A TextBox's Text property is registered like shown above, i.e. with the BindsTwoWayByDefault flag.

---

At the TextBox Binding in the UserControl's XAML you may also want to update the source property while the user is typing (instead of only on lost focus):

<TextBox Text="{Binding HmiField,
                ElementName=usercontrol,
                UpdateSourceTrigger=PropertyChanged}"/>

or without the otherwise useless generated usercontrol field:

<TextBox Text="{Binding HmiField,
                RelativeSource={RelativeSource AncestorType=UserControl}
                UpdateSourceTrigger=PropertyChanged}"/>