class MyDatagridView: DataGridView
    {

    }

/// <summary>
/// Отслеживаем нажатие правой кнопкой мышки по гриду
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void dataGridView_Main_MouseDown(object sender, MouseEventArgs e)
{
    DataGridView.HitTestInfo Hti;

    if (e.Button == MouseButtons.Right)
    {
        Hti = dataGridView_Main.HitTest(e.X, e.Y);
        if (Hti.Type == DataGridViewHitTestType.Cell)
        {
            if (!((DataGridViewRow)(dataGridView_Main.Rows[Hti.RowIndex])).Selected)
            {
                dataGridView_Main.ClearSelection();
                ((DataGridViewRow)dataGridView_Main.Rows[Hti.RowIndex]).Selected = true;
            }
        }
    }
}