<Window x:Class="Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="358" Width="462" x:Name="Window1">
 
    <Canvas>
        <Button Height="52" x:Name="Button1"  Width="157" _
          RenderTransformOrigin="0.5,0.5"  Content="Button" Canvas.Top="149" _
          Canvas.Left="127"/>
    </Canvas>   
</Window>

Partial Public Class Window1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.Windows.RoutedEventArgs) Handles Button1.Click
        Dim Fenster As New Window2
        Fenster.ShowDialog()
    End Sub
 
End Class

<Window x:Class="Window2"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window2" Height="300" Width="300">
    <Grid>
        <Button Height="43" HorizontalAlignment="Left" Margin="25,50,0,0" _
          Name="Button1" VerticalAlignment="Top" Width="91">Button</Button>
    </Grid>
</Window>

Partial Public Class Window2
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.Windows.RoutedEventArgs) Handles Button1.Click
        Window1.Button1.Content = "neuer text"
    End Sub
End Class