Question

Why doesn’t this binding update?

code:
MainWindow.xaml

<Window x:Class="WpfApplication12.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication12"
        Height="350" Width="525">
    <StackPanel>
        <local:UserControl1 x:Name="usr" />
        <TextBlock Text="{Binding ElementName=usr, Path=txt.Text}" />
    </StackPanel>
</Window>

UserControl1.xaml

<UserControl x:Class="WpfApplication12.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <TextBox Text="qwe" x:Name="txt" />
</UserControl>

Answer 1

The TextBox inside the UserControl is inaccessible due to its protection level, also it is a field, you can never bind to those. You would need to expose it in the code behind of the UserControl as public property.

public TextBox Txt
{
    get { return txt; }
}

Edit: As Henk Holterman pointed out you might not want to expose the whole TextBox, so you could define a dependency property to which the TextBox internally binds for example.