From Evernote:

[VB.NET]ﾃｷｽﾄボックスで指定の桁数を入力したら次へフォーカス

# Region " ﾃｷｽﾄﾎﾞｯｸｽ７桁でフォーカス移動"

     Private Sub txtTEST_KeyPress( ByVal sender As System.Object, _ 

            ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtTEST.KeyPress

        Dim strBefore As String = Me .txtTEST.Text.Trim

        If strBefore.Length = 6 AndAlso _

            Asc(e.KeyChar) <> Keys.Back AndAlso _

                e.KeyChar.ToString.Trim.Length = 1 AndAlso _

                    Me.txtEigen.SelectionStart = 6 Then

            Me.txtNEXT.Select()

        End If

    End Sub

# End Region