Class Window1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.Windows.RoutedEventArgs)
        Dim Fenster As New Window2
        AddHandler Fenster.mChangeText, AddressOf Me.ChangeText
        Fenster.ShowDialog()
    End Sub
 
    Private Sub ChangeText(ByVal text As String)
        Button1.Content = text
    End Sub
 
End Class

Partial Public Class Window2
 
    Public Event mChangeText(ByVal text As String)
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.Windows.RoutedEventArgs)
        RaiseEvent mChangeText(Me.TextBox1.Text)
    End Sub
 
End Class

 <Button x:Name="Button1" ... Click="Button1_Click"/>