绑定到UserControl的属性不起作用

Question

I got a custom TextBox which I plan to include in another UserControl, however when setting up the Binding for it, it simply just doesn't bind.

I simplified the code for clarity.

My custom TextBox:

<UserControl DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <TextBox Text="{Binding Text, UpdateSourceTrigger=PropertyChanged}" />
</UserControl>

partial class CustomTextBox : UserControl 
{
    public string Text
        {
            get { return (string)GetValue(TextProperty); }
            set
            {
                SetValue(TextProperty, value);
            }
        }

        public static readonly DependencyProperty TextProperty = DependencyProperty.Register(
            "Text",
            typeof(string),
            typeof(CustomTextBox),
            new PropertyMetadata(String.Empty));
}

This binding works as expected. When using CustomTextBox in another UserControl or Window, I can access the property just as expected.

The following code blocks describe the UserControl that uses CustomTextBox and the corresponding ViewModel with the property I want to bind Text to.

<UserControl>
    <UserControl.DataContext>
        <vm:MyViewModel />
    </UserControl.DataContext>
    <local:CustomTextBox Text="{Binding FooBar, UpdateSourceTrigger=PropertyChanged}" />
</UserControl>

public class MyViewModel : INotifyPropertyChanged
{
    private string _fooBar;
    public string FooBar
        {
            get { return _fooBar = (_fooBar ?? ""); }
            set
            {
                _fooBar = value; OnPropertyChanged();
            }
        }

    public event PropertyChangedEventHandler PropertyChanged;
}

My problem occurs exactly when I want to bind the Text property to a ViewModel in another UserControl, it just doesn't work. In this case I tried to bind the Text property to the FooBar property on the MyViewModel class, however changes to the Text property do not get reflected on the FooBar property and vice-versa. However when I hover over the binding in the XAML view, it shows the type of the property, so I don't exactly see what's wrong here.
My best guess is that it has to do with two bindings accessing the same property.

Answer 1

modify DP registration to include FrameworkPropertyMetadataOptions.BindsTwoWayByDefault option

public static readonly DependencyProperty TextProperty = DependencyProperty.Register(
    "Text",
    typeof(string),
    typeof(CustomTextBox),
    new FrameworkPropertyMetadata(String.Empty, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));