Public Class uc_Textbox
    Inherits System.Windows.Controls.TextBox
 
    Private Sub uc_Textbox_MouseDown(ByVal sender As Object, ByVal e As _
      System.Windows.Input.MouseButtonEventArgs) Handles Me.MouseDown
        Dim x As New uc_TextboxSub
        '...
    End Sub
 
End Class
 
...Klasse 2
 
Public Class uc_TextboxSub
    Inherits System.Windows.Controls.TextBox
 
    Public Sub New()
        Me.Text = "Test sub"
        Me.Background = Brushes.Blue
    End Sub
 
End Class