求解答，如何解决服务器无法在发送 HTTP 标头之后设置内容类型问题

 [PostAuthorityAttribute]

        public string btnExcel(FormCollection form, string Tname)

        {

            string a = "";

            try

            {

                if (HttpContext.Request["year"] == null || HttpContext.Request["year"] == "")

                    return null;

                int depid = Convert.ToInt32(Session["DepartId"]);

                int userid = Convert.ToInt32(Session["userid"]);

                int year = Convert.ToInt32(HttpContext.Request["year"]);

                string sql = "";

                string modelPath = "";

                int index = 0;

                string[] arr = Tname.Split(',');

                for (int i = 0; i < arr.Length; i++)

                {

                    switch (arr[i])

                    {

                        case "利润预算表":

                            sql = " SELECT [year],[month],[yysr],[zyywsr],[qtywsr] ,[yycb] ,[zyywcb],[qtywcb] ,[yysjjfj] ,[xsfy],[glfy] ,[cwfy] ,[zcjzss],[gyjzbdsy],[tzsy] ,[lhtzsy] ,[yylr],[yywsr],[yywzc],[lrze] ,[sdsfy],[jlr]  FROM V_Lrys_2016 where depid=" + depid + " and ( (years=" + year + " and months not in (1,2,3,-4)) or (years=" + Convert.ToInt32(year + 1) + " and months in (1,2,3,-4)) )";

                            modelPath = "Sclrysb.xls";

                            index = 1;

                            break;

                        case "现金流量预算表":

                            sql = " SELECT [year],[month],[tglwj],[sfhf],[jyhdxj],[jynbxjlr] ,[jyxjlr],[sjlrlr],[qtlr],[jyxjlrxj],[jslwj],[zgj],[zfsf],[zfqt],[jynbxjlc],[jylc],[sjlc],[qtlc],[jyxjlcxj],[jyhdje],[shxj],[qdxj],[czje],[czzgsje],[sdxj],[tznbxjlr],[tzxjlrxj],[gjxj],[tzxj],[qdzgsxj],[zfqtxj],[tznbxjlc],[tzxjlcxj],[tzje],[xstzj],[jkj],[qtczj],[cznbxjlr],[czxjlrxj],[chj],[fpj],[czqtj],[cznbxjlc],[czxjlcxj],[czhdje],[djwzje],[qcdjwye],[qmdjwye]  FROM [Planning].[dbo].[V_Xjllb_2016] where zt  = 0 and depid =" + depid + " and ( year=" + year + " or year=" + Convert.ToInt32(year + 1) + " or year='')";

                            sql += " AND (([year] = " + year + " AND [month] IN (4,5,6,7,8,9,10,11,12)) OR ([year] = " + year + "+1" + " AND [month] IN(1,2,3)) or ( [month] IN ('第一季度小计','第二季度小计','第三季度小计','第四季度小计','年度合计')))";

                            modelPath = "TXjllysfb_2016.xls";

                            index = 2;

                            break;

                        case "资产负债预算表":

                            sql = " SELECT [year],[month] ,[hbzj] ,[jyxjrzc],[yspj],[yszk],[yfkx],[yslx],[ysgl],[qtysk],[ch],[ynndqdfldzc],[ddfy],[qtldzc],[dclldzcsy],[nbwl],[ldzchj],[kgcsjrzc],[cyzdqtz],[cqysk] ,[cqgqtz],[tzxfdc],[gdzcyz],[ljzj],[gdzcjz],[zjgc] ,[gcwz] ,[gdzcql] ,[scxswzc] ,[yqzc] ,[wxzc] ,[ljtx] ,[wxzcjz] ,[kfzc] ,[sy] ,[cqdtfy] ,[dysdszc] ,[qtfldzc] ,[fldzchj] ,[zczj] ,[dqjk] ,[jyxjrfz] ,[yfpj] ,[yfzk] ,[yskx] ,[yfzgxc] ,[yjsf] ,[yflx] ,[yfgl] ,[qtyfk] ,[ytfy] ,[ynndqdfldfz] ,[qtldfz] ,[ldfzhj] ,[cqjk] ,[yfzj] ,[cqyfk] ,[zxyfk] ,[yjfz] ,[dysdsfz] ,[qtfldfz] ,[fldfzhj] ,[fzhj] ,[sszb] ,[zbgj] ,[kcg],[zxcc] ,[yygj],[wfplr] ,[gsymgssyzqyhj] ,[ssgdqy],[syzqyhj] ,[fzhsyzqyhj] ,[zczeyfzhsyzqyzj] ,[zcfzbhxjllb],[zcfzbhlrb]  FROM V_Tzcfys_2016  WHERE  depid=" + depid + "  and ( (years=" + year + " and months not in (1,2,3,-4)) or (years=" + Convert.ToInt32(year + 1) + " and months in (1,2,3,-4)) )";

                            modelPath = "Tzcfzys_2016.xls";

                            index = 2;

                            break;

                    }

                    DataTable dt = DbHelperSQL.Query(sql.ToString()).Tables[0];



                    string excelName =arr[i] + DateTime.Now.ToString("yyyyMMddHHmmssms") + ".xls";

                    NpoiHelper npoi = new NpoiHelper();

                    npoi.ToExcel2(dt, modelPath, excelName, index);

                    a = "Success";

                }

            }

            catch (Exception ex)

            {

                LogHelper.Log(ex);

            }

            return a;