winform_dataGridView更新问题,谢谢！

为什么这样可以，注释的那部分方法不行？出错。谢谢回答。
            int j = dataGridView1.SelectedRows.Count;
            int[] l = new int[j];            int i;
            for (i = 0; i < j; i++)
            {
                l[i] = dataGridView1.SelectedRows[i].Index;
            }            int k = 0;
            while (k < j)
            {
                this.northwindDataSet.Customers.Rows[l[k]].Delete();
                k++;
            }
            //this.customersTableAdapter.Update(this.northwindDataSet.Customers);            //int count = dataGridView1.SelectedRows.Count;
            //for (int i = 0; i < count; i++)
            //{
            //    this.northwindDataSet.Customers.Rows.RemoveAt(dataGridView1.SelectedRows[i].Index);
            //}
            //foreach (DataGridViewRow dr in dataGridView1.Rows)
            //{
            //    if (dr.Selected)
            //    {
            //        this.northwindDataSet.Customers.Rows[dr.Index].Delete();
            //    }
            //}
            try
            {
                this.customersTableAdapter.Update(this.northwindDataSet.Customers);
            }
            catch (Exception e1)
            {
                MessageBox.Show(e1.Message);
            }