C#从datagridview中读取某列的所有字符串，并把它们拼成一个字符串

Question

StringBuilder detail = new StringBuilder("");

    string f10username;
    string f10number;
    public  void button1_Click(object sender, EventArgs e)
    {
        if (a + b + c + d == 4)
        {

        for (int i = 0; i < this.dataGridView1.Rows.Count; i++)
        {
            f10username = this.dataGridView1.Rows[i].Cells[1].Value.ToString();
            f10number = this.dataGridView1.Rows[i].Cells[9].Value.ToString();

           detail.Append("图书编号" + f10username + "有" + f10number + "本;\n");

        }