Can't access a wpf control from an outside class - wpf

Question

I want to call a label control on my main window from an outside class. But the class doesn't recognize it.
My file structure is like this
ZoomBorder.cs
MainWindow.xaml
XAML:
<Window x:Class="GUI.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:PanAndZoom"
Title="PanAndZoom" Height="600" Width="900" WindowStartupLocation="CenterScreen">
<Grid>
<Grid.RowDefinitions>
<RowDefinition></RowDefinition>
<RowDefinition Height="100"></RowDefinition>
</Grid.RowDefinitions>
<local:ZoomBorder x:Name="border" ClipToBounds="True" Background="Gray">
<!--<Image Source="/GUI;component/Images/Desert.jpg"/>-->
<Canvas Width="300" Height="300" Background="Green"></Canvas>
</local:ZoomBorder>
<StackPanel Grid.Row="1">
<Label x:Name="pos" x:FieldModifier="public">Position</Label>
</StackPanel>
</Grid>
</Window>
ZoomBorder.cs:
public class ZoomBorder : Border
{
...
private void SomeMethod()
{
// this doesn't work!
pos.Content = "This label is changed by ZoomBorder!";
}
}

Answer 1

The way you tried is only works if the property defined in the same class :
pos.Content = "This label is changed by ZoomBorder!";
In case of static property from other class, you can access it this way :
MainWindow.pos = ....
//or generally
ClassName.PropertyName = ....
Unfortunately UI controls in your XAML isn't static property. In case of non static property from other class, you need class instance to access the property :
MainWindow mainWindowInstance = new MainWindow();
mainWindowInstance.pos = ....
Snippet above is only example, in your case you'll need to find a way to pass current MainWindow instance displayed to ZoomBorder instead of creating new instance as this example demonstrates.

Answer 2

There's no reason for it to work. What you're trying to do is pretty much equivalent to :
class Parent // = MainWindow
{
object xxx;
Child child;
}
class Child // = ZoomBorder
{
void SomeFunction()
{
this.xxx = ...; // Doesn't work, the class Child doesn't have any xxx field
}
}

Answer 3

This should work.
var wnd = Application.Current.MainWindow as MainWindow;
var label = wnd.pos;