public static string str = "";
        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            if (this.dataGridView1.SelectedRows.Count > 0)
                str = this.dataGridView1.SelectedRows[0].Cells[0].Value.ToString();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            DialogResult dr = MessageBox.Show("确定要删除吗？", "提示", MessageBoxButtons.OKCancel);
            if (dr == DialogResult.OK)
            {
                if (str == "")
                {
                    MessageBox.Show("请选择你要删除的行！", "警告", MessageBoxButtons.OKCancel);
                    return;
                }
                string sql = "delete from text1 where bh='" + str + "'";
                str = "";
                MessageBox.Show("该学生已被删除！", "提示");
            }
str 没有取得值，一直弹出“请选择你要删除的行”，不知道怎么改