WPF usercontrol Twoway binding Dependency Property

Question

I created a Dependency Property in the usercontrol, but however changes in the usercontrol was NOT notified to the Viewmodel

Usercontrol

<UserControl x:Class="DPsample.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-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
<Grid>
    <TextBox x:Name="txtName"></TextBox>
</Grid>

UserControl.cs

/// <summary>
/// Interaction logic for UserControl1.xaml
/// </summary>
public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();
    }

    #region SampleProperty 

    public static readonly DependencyProperty SamplePropertyProperty
                                            = DependencyProperty.Register("SampleProperty", 
                                            typeof(string), 
                                            typeof(UserControl1), 
                                            new PropertyMetadata(OnSamplePropertyChanged));


    public string SampleProperty
    {
        get { return (string)GetValue(SamplePropertyProperty); }
        set { SetValue(SamplePropertyProperty, value); }
    }


    static void OnSamplePropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        (obj as UserControl1).OnSamplePropertyChanged(e);
    }
    private void OnSamplePropertyChanged(DependencyPropertyChangedEventArgs e)
    {
        string SamplePropertyNewValue = (string)e.NewValue;

        txtName.Text = SamplePropertyNewValue;
    }

    #endregion
}

MainWindow

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:DPsample" x:Class="DPsample.MainWindow"
    Title="MainWindow" Height="350" Width="525">
<Grid>

    <local:UserControl1 SampleProperty="{Binding SampleText,Mode=TwoWay}" HorizontalAlignment="Left" Margin="76,89,0,0" VerticalAlignment="Top" Width="99"/>
    <Button Content="Show" HorizontalAlignment="Left" Margin="76,125,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>

</Grid>

MainWindow.cs

 public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new MainViewModel();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        var item = this.DataContext as MainViewModel;
        MessageBox.Show(item.SampleText.ToString());
    }

MainViewModel.cs

public class MainViewModel : NotifyViewModelBase
{
    public MainViewModel()
    {
        this.SampleText = "test";           
    }

    private string _sampleText;
    public string SampleText
    {
        get { return _sampleText; }
        set { _sampleText = value; OnPropertyChanged("SampleText"); }
    }
}

You haven't coded anything that passes changes of the TextBox.Text property to SampleProperty. You should also use a binding there, with RelativeSource={RelativeSource AncestorType=UserControl}. — Clemens
– Clemens, Commented Sep 23, 2014 at 7:16
@Clemens do i need to use the viewmodel for the usercontrol? — Spen D
– Spen D, Commented Sep 23, 2014 at 7:23
Not in this case. However, the UserControl might as well directly bind to your view model's SampleText property. Then there would be no need at all for SampleProperty. — Clemens
– Clemens, Commented Sep 23, 2014 at 7:28

Clemens · Accepted Answer · 2020-09-24 07:56:46Z

67

Bind the TextBox.Text property in the UserControl to its SampleProperty like this:

<TextBox Text="{Binding SampleProperty,
                RelativeSource={RelativeSource AncestorType=UserControl}}"/>

Now you could simply remove your OnSamplePropertyChanged callback.

You might also register SampleProperty to bind two-way by default like this:

public static readonly DependencyProperty
    SamplePropertyProperty = DependencyProperty.Register(
        "SampleProperty", typeof(string), typeof(UserControl1),
        new FrameworkPropertyMetadata(
            null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

edited Sep 24, 2020 at 7:56

answered Sep 23, 2014 at 7:26

Clemens

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1 Comment

Diogo Mendonça Over a year ago

Can't believe I've read so many tutorials and examples on custom UserControls and DependencyProperties and not one mentions the FrameworkPropertyMetadata that solves passing the value back to the model. Great job man!

Clemens · Accepted Answer · 2018-05-11 17:42:27Z

4

An alternative way to do this is an ElementName Binding. First assign the x:Name attribute to the UserControl (for example x:Name="MyUC"), then change the binding to:

<TextBox Text="{Binding ElementName=MyUC, Path=SampleProperty}"/>

edited May 11, 2018 at 17:42

Clemens

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answered Apr 2, 2017 at 21:54

Dabbas

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