My Dependency Property is only binding one way

Question

I'm new to WPF, and I'm doing something which I feel is pretty straightforward. I made a UserControl that contains a textbox, and I want to bind the TextBox.Text property to a dependency property of my usercontrol, Value. The textbox correctly displays Value, and if you change the value of Value, the textbox updates accordingly. But, if I change the value in the Textbox, Value does not change to reflect it. Even if I manually set the binding Mode to two-way, it still only binds one way. I made Value a dependency property, so shouldn't that take care of things?

Here's my XAML (I stripped away the other controls for the sake of readability):

<UserControl x:Class="WindowsApp.FormBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
             xmlns:local="clr-namespace:WindowsApp">
    <Grid DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=local:FormBox}}">
        <TextBox x:Name="TextForm" Text="{Binding Path=Value, Mode=TwoWay}" />
    </Grid>
</UserControl>

And here's the c#, Visual Studio auto-generated it for me so maybe there's something not right about it

public String Value
{
    get { return (String)GetValue(ValueProperty); }
    set 
    {
        SetValue(ValueProperty, value);
    }
}
public static readonly DependencyProperty ValueProperty =
    DependencyProperty.Register("Value", typeof(String), typeof(FormBox));

In other parts of the code, I just assign Value in XAML by just doing Value="{Binding Whatever}" Any advice or help would be appreciated. Thank you.

Answer 1

You need to set the Text binding's UpdateSourceTrigger to PropertyChanged. The default for TextBox is LostFocus. If this is the only control in a window then there isn't anything else that can receive focus, so a LostFocus event never fires and the binding never updates the source.

<TextBox x:Name="TextForm" Text="{Binding Path=Value, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" />