C# 从c中的最后一行开始在gridview上循环

我正在创建学生组。我在c asp.net中有一个Gridview。它包含一个项目模板字段标签。我有一个dropdownlist，从中选择要创建的组数。我希望在Gridview上应用循环，以便在我从dropdownlist中选择任何整数值时，项目模板字段标签获取值例如 如果我从dropdownlist中选择3，则循环工作，因为它首先将值分配给前3名学生，然后移动到gridview的最后一个，并将值从底部分配给前3名学生，然后再次从顶部移动，依此类推

name       groupno.

A             1   
B             2
C             3
D             1
E             2
F             3 
G             1
H             3
I             2
J             1
K             3
L             2
M             1

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请帮助我。

粘贴一些代码，以便更好地理解

protected void Button1_Click(object sender, EventArgs e)
{
        DataTable dt1;
        DataRow row; 
        dt1 = new DataTable();
        dt1.Columns.Add("GroupNo");
        dt1.Columns.Add("Registration");
        dt1.Columns.Add("Name");
        dt1.Columns.Add("Marks");
        dt1.Columns.Add("Technology");
        dt1.AcceptChanges();
        return dt1;

        int starting = 0, Ending = GridView1.Rows.Count-1, flag = 0, groupsize =5 , Count_Stu = 1;

        for (int i = 0; i <= countvalue; i++)
        {
            if (flag == 0)  // Top To Down
            {
                row = dt1.NewRow();
                row["GroupNo"] = Count_Stu.ToString();
                row["Registration"] = GridView1.Rows[starting].Cells[0].Text;
                row["Name"] = GridView1.Rows[starting].Cells[1].Text;
                row["Marks"] = GridView1.Rows[starting].Cells[2].Text;
                row["Technology"] = GridView1.Rows[starting].Cells[3].Text;
                dt1.Rows.Add(row);
                Count_Stu++;
                starting++;
            }
            else if (flag == 1) // Down To Up
            {
                row = dt1.NewRow();
                row["GroupNo"] = Count_Stu.ToString();
                row["Registration"] = GridView1.Rows[Ending].Cells[0].Text;
                row["Name"] = GridView1.Rows[Ending].Cells[1].Text;
                row["Marks"] = GridView1.Rows[Ending].Cells[2].Text;
                row["Technology"] = GridView1.Rows[Ending].Cells[3].Text;
                dt1.Rows.Add(row);
                Count_Stu++;
                Ending--;
            }

            if (Count_Stu == groupsize+1)   //Reset 
            {
                if (flag == 0)
                    flag = 1;
                else
                    flag = 0;
                Count_Stu = 1;
            }
        }
        GridView2.DataSource = dt1;
        GridView2.DataBind();

    }