wpf, binding data to child of another usercontrol - wpf

Question

i am trying to bind a Textblock's Text property of MainWindow to a Listbox's Items.Count of another Usercontrol, and the textblock of Mainwindow failed to read the value of binding source.
but if both textblock and listbox belong to the same usercontrol or window, the binding is fine.
i know i can define a property of in the usercontrol and make it exposed to other framework elements(e.g. textblock) of Mainwindow, and then it can be bond.
i just cannot understand why the binding to the listbox of another usercontrol failed. any suggestion will be greatly appreciated.
here below is the sample i made for better explanation.
UserControl Xaml:
<UserControl x:Class="stackoverFlow.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"
xmlns:local="clr-namespace:stackoverFlow"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800">
<StackPanel>
<ListBox x:Name="lbDemo">
<ListBoxItem>
<TextBlock Text="element 01"/>
</ListBoxItem>
<ListBoxItem>
<TextBlock Text="element 02"/>
</ListBoxItem>
<ListBoxItem>
<TextBlock Text="element 03"/>
</ListBoxItem>
</ListBox>
<!--binding to the control of the same usercontrol is fine-->
<TextBlock Text="{Binding ElementName=lbDemo,Path=Items.Count,StringFormat='there are {0} items in the listbox.'}"/>
</StackPanel>
</UserControl>
MainWindow Xaml:
<Window x:Class="stackoverFlow.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:stackoverFlow"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<StackPanel>
<local:UserControl1 x:Name="userControlDemo"/>
<Separator Margin="0 5 0 15"/>
<!--binding failed-->
<TextBlock Text="{Binding ElementName=userControlDemo,Path=lbDemo.Items.Count, StringFormat='there are {0} items in the listbox of Usercontrol1'}"/>
</StackPanel>
</Window>

Answer 1

In your User control, you should add a get accessor like this:
public int ListCount {
get {
return this.lbDemo.Items.Count;
}
}
And in your Main Window, you call it like this:
<TextBlock Text="{Binding ElementName=userControlDemo, Path=ListCount, StringFormat='there are {0} items in the listbox of Usercontrol1'}" />