【求助】关于Excel内容导入数据库时，SQL语句与外键约束发生冲突。在线跪等。

SqlConnection conn = new SqlConnection(System.Configuration.ConfigurationSettings.AppSettings["mystring"]);

    protected void Page_Load(object sender, EventArgs e)
    {


    }

    public DataSet ExecleDs(string filenameurl, string table)
    {
        string strConn = "Provider=Microsoft.Jet.OleDb.4.0;" + "data source=" + filenameurl + ";Extended Properties='Excel 8.0; HDR=YES; IMEX=1'";
        OleDbConnection con = new OleDbConnection(strConn);
        con.Open();
        DataSet ds = new DataSet();
        OleDbDataAdapter odda = new OleDbDataAdapter("select * from [Sheet1$]", con);
        odda.Fill(ds, table);
        return ds;
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (FileUpload1.HasFile == false)
        {
            Response.Write("<script>alert('请您选择Excel文件')</script> ");
            return;
        }
        string IsXls = System.IO.Path.GetExtension(FileUpload1.FileName).ToString().ToLower();
        if (IsXls != ".xls")
        {
            Response.Write("<script>alert('只可以选择Excel文件')</script>");
            return;
        }

        conn.Open();

        string filename = DateTime.Now.ToString("yyyymmddhhMMss") + FileUpload1.FileName;
        string savePath = Server.MapPath(("~\\upfiles\\") + filename);
        FileUpload1.SaveAs(savePath);
        DataSet ds = ExecleDs(savePath, filename);
        DataRow[] dr = ds.Tables[0].Select();
        int rowsnum = ds.Tables[0].Rows.Count;
        if (rowsnum == 0)
        {
            Response.Write("<script>alert('Excel表为空表,无数据!')</script>");
        }
        else
        {
            for (int i = 0; i < dr.Length; i++)
            {

                string student_xh = dr[i]["学号"].ToString();//日期 excel列名【名称不能变,否则就会出错】
                string student_xm = dr[i]["姓名"].ToString();//编号列名以下类似
                string student_jg = dr[i]["籍贯"].ToString();
                string student_xb = dr[i]["性别"].ToString();
                string student_sr = dr[i]["出生日期"].ToString();
                string student_bj = dr[i]["班级号"].ToString();
                string student_dh = dr[i]["电话号码"].ToString();
                string student_xf = dr[i]["已修学分"].ToString();
                string student_rx = dr[i]["入学时间"].ToString();
                string student_zz = dr[i]["家庭住址"].ToString();
                string student_mm = dr[i]["密码"].ToString();
                string student_bz = dr[i]["备注"].ToString();

                string sqlcheck = "select count(*) from student where 学号 = ' " + student_xh + "'";

                SqlCommand sqlcmd = new SqlCommand(sqlcheck, conn);
                int count = Convert.ToInt32(sqlcmd.ExecuteScalar());
                if (count < 1)
                {
                    string insertstr = "insert into student (学号,姓名,籍贯,性别,出生日期,班级号,电话号码,已修学分,入学时间,家庭住址,密码,备注) values('" + student_xh + "','" + student_xm + "','" + student_jg + "','" + student_xb + "','" + student_sr + "','" + student_bj + "','" + student_dh + "','" + student_xf + "','" + student_rx + "','" + student_zz + "','" + student_mm + "','" + student_bz + "')";

                    SqlCommand cmd = new SqlCommand(insertstr, conn);
                    try
                    {
                        int a = cmd.ExecuteNonQuery();
                    }
                    catch (MembershipCreateUserException ex)
                    {
                        Response.Write("<script>alert('导入内容:" + ex.Message + "')</script>");
                    }
                }
                else
                {
                    Response.Write("<script>alert('内容重复！禁止导入');location='DateInfo.aspx'</script></script> ");
                    continue;
                }
            }
            Response.Write("<script>alert('Excle表导入成功!');location='DateInfo.aspx'</script>");
        }

        conn.Close();

    }

在向student表插入时，外键冲突，与另外一张表class中的主键班级号  发生冲突    代码要怎么判断呢？ --------------------编程问答-------------------- 把班级号全取出来   存进 list        遍历excel数据时  判断 list中是否包含这个班级号
如果list中不包含  跳出循环继续下一条

补充：.NET技术 ,  C#