DataGridの選択マークの制御について

'対象となるDataGridViewのあるフォームのロード
    Private Sub EditValue_Load(ByVal eventSender As System.Object, ByVal eventArgs As System.EventArgs) Handles MyBase.Load
        '最初にこの命令を実行すると行ヘッダーの三角を消せる
        DataGridView1.RowTemplate.HeaderCell = New myDataGridViewRowHeaderCell

        DataGridView1.Rows.Add()
        DataGridView1.Rows.Add()
    End Sub


'行ヘッダーの三角マークを消す処理のためにClassを作る
Public Class myDataGridViewRowHeaderCell
    Inherits DataGridViewRowHeaderCell
    '行ヘッダーの三角マークを消す処理
    Protected Overrides Sub Paint(ByVal graphics As Graphics, _
                                ByVal clipBounds As Rectangle, _
                                ByVal cellBounds As Rectangle, _
                                ByVal rowIndex As Integer, _
                                ByVal cellState As DataGridViewElementStates, _
                                ByVal value As Object, _
                                ByVal formattedValue As Object, _
                                ByVal errorText As String, _
                                ByVal cellStyle As DataGridViewCellStyle, _
                                ByVal advancedBorderStyle As DataGridViewAdvancedBorderStyle, _
                                ByVal paintParts As DataGridViewPaintParts)

        MyBase.Paint(graphics, clipBounds, cellBounds, _
                    rowIndex, cellState, value, formattedValue, _
                    errorText, cellStyle, advancedBorderStyle, _
                    paintParts And Not DataGridViewPaintParts.ContentBackground)
    End Sub

End Class