WinForm DataGridView根据选中的行多删

private void btndel_Click(object sender, EventArgs e)
        {
            int count = 0;
            //获取选中的行数
            count = this.dataGridView1.SelectedRows.Count;
            if (count == 0)
            {
                MessageBox.Show("至少选择一项！！", "提示");
                return;
            }
            DialogResult dr = MessageBox.Show("共"+count+"条，确定要删除吗？","警告",MessageBoxButtons.YesNo,MessageBoxIcon.Information);
            if(dr==DialogResult.Yes)
            {

                string str = "";
                //循环选中的行并获取想要的值
                foreach (DataGridViewRow row in dataGridView1.SelectedRows)
                {
                    str += row.Cells[0].Value.ToString()+",";
                }
                if (str.Length > 0)
                {
                    str = str.Substring(0, str.Length - 1);
                }
                StudentBLL bll = new StudentBLL();
                //调用删除的方法
                if (bll.DelStudent(str) > 0)
                {
                    MessageBox.Show("删除成功", "提示");
                    StudentShow();//刷新数据
                }
                else
                {
                    return;
                }
            }