HttpContext modification breaks SharePoint Site

Often it is required to return a file from a web-part OnSubmit(). The standard practise is:

HttpContext.Current.Response.Clear();
HttpContext.Current.Response.AddHeader("content-disposition", String.Format("attachment; filename={0}",_ExcelFile));
HttpContext.Current.Response.Charset = "";
HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);
HttpContext.Current.Response.ContentType = "application/ms-excel";

However, once the user clicks on the Submit, the SharePoint Site will stop responding. Now there are ways to fix this, either you do a Server Redirect and create a new HttpContext or just add the following line to your Button:

btnSubmit.OnClientClick = "this.form.onsubmit = function() {return true;}";

So in your WebPart btnSubmit_OnClick(),

private void btnSubmit_Click(object sender, EventArgs e)
        {
            if (IsEditMode) return;
            EnsureChildControls();
            System.IO.StringWriter oStringWriter = new System.IO.StringWriter();
            System.Web.UI.HtmlTextWriter oHtmlTextWriter = new System.Web.UI.HtmlTextWriter(oStringWriter);
            try
            {
                if (dtStartDate.IsDateEmpty || dtEndDate.IsDateEmpty) return;
                String strCashType = ddCashType.Text;
                String strTType = ddTransactionType.Text;
                String xlsFileName = DateTime.Now.ToString("yyyyMMddHHmmss") + ".xls";
                if (ExcelFile == null || ExcelFile == " ")
                    _ExcelFile = "Export" + xlsFileName;
                else
                    _ExcelFile += xlsFileName;
                //
                HttpContext.Current.Response.Clear();
                HttpContext.Current.Response.AddHeader("content-disposition", String.Format("attachment; filename={0}",_ExcelFile));
                HttpContext.Current.Response.Charset = " ";
                HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);
                HttpContext.Current.Response.ContentType = "application/ms-excel";
                //

                DataGrid dgTemp = new DataGrid();
                dgTemp.DataSource = oSycoDB.getSycoData(dtStartDate.SelectedDate.ToString("yyyy-MM-dd"),
                                                        dtEndDate.SelectedDate.ToString("yyyy-MM-dd"),
                                                        strCashType, strTType);
                dgTemp.DataBind();
                dgTemp.RenderControl(oHtmlTextWriter);
                HttpContext.Current.Response.Write(oStringWriter.ToString());
                HttpContext.Current.Response.Flush();
                HttpContext.Current.Response.End();
            }
            catch (Exception ex)
            {
                //
                HttpContext.Current.Response.Clear();
                HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=Error.txt");
                HttpContext.Current.Response.Charset = " ";
                HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);
                HttpContext.Current.Response.ContentType = "text/plain";
                oHtmlTextWriter.Write(ex.Message+"\n"+ex.StackTrace);
                HttpContext.Current.Response.Write(oStringWriter.ToString());
                HttpContext.Current.Response.Flush();
                HttpContext.Current.Response.End();
                //
                SingleJunket.LogMessage(ex.Message, EventLogEntryType.Error,1002, 1);
            }
        }

Enjoy coding.