Public Sub btn_Click(sender As Object, e As EventArgs)
 
        Dim ctrl() As Control = Me.Controls.Find("TheBox", True)
 
        If ctrl.Length > 0 AndAlso TypeOf ctrl(0) Is TextBox Then
            Dim tb As TextBox = DirectCast(ctrl(0), TextBox)
            tb.Text = "Da tut sich schon etwas"
        End If
 
    End Sub