Server cannot add header after sending C # excel inline HTTP headers

Question

I have this problem for trying to return an excel inline response.

I searched all the help pages for answers to them and cannot find something that works.

I made a simplified example of what I am doing below. This question always happens.

The error in this line is:

Line 56:         Response.AddHeader("content-disposition",

Code behind:

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        PleaseWait();
        Call();
    }

    protected void Call()
    {
        strSql = "SELECT * FROM ....... ; ";

        SqlCommand cmd = new SqlCommand(strSql);
        DataTable dt = GetData(cmd);

        GridView GridView1 = new GridView();
        GridView1.AllowPaging = false;
        GridView1.DataSource = dt;
        GridView1.DataBind();

        Response.Clear();
        Response.Buffer = true;
        Response.BufferOutput = true;
        Response.ClearHeaders();

        Response.AddHeader("content-disposition",
         "attachment;filename= "Output.xls");
        Response.Charset = "";
        Response.ContentType = "application/vnd.ms-excel";
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);

        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
            GridView1.Rows[i].Attributes.Add("class", "textmode");
        }
        GridView1.RenderControl(hw);

        string style = @"<style> .textmode { mso-number-format:\@; } </style>";
        Response.Write(style);
        Response.Output.Write(sw.ToString());
        Response.Flush();
        Response.End();
    }

    protected void PleaseWait()
    {
        this.Response.Write("<meta http-equiv=X-UA-Compatible content=IE=8>");
        this.Response.Write(@"<style type=text/css media=all>");
        this.Response.Write(@".loading");
        this.Response.Write(@"{");
        this.Response.Write(@"text-align: center;");
        this.Response.Write(@"padding-top: 30px;");
        this.Response.Write(@"border-width: 1px solid #000;");
        this.Response.Write(@"width: 300px;");
        this.Response.Write(@"height: 100px;");
        this.Response.Write(@"-ms-filter: alpha(opacity=90);");
        this.Response.Write(@"-ms-opacity: 0.90;");
        this.Response.Write(@"border-style: solid;");
        this.Response.Write(@"background-color: #FFFFFF;");
        this.Response.Write(@"position: absolute;");
        this.Response.Write(@"font-family: Trebuchet MS;");
        this.Response.Write(@"font-size: small;");
        this.Response.Write(@"position: absolute;");
        this.Response.Write(@"top: 0;");
        this.Response.Write(@"bottom: 0;");
        this.Response.Write(@"left: 0;");
        this.Response.Write(@"right: 0;");
        this.Response.Write(@"margin: auto;");
        this.Response.Write(@"display: block;");
        this.Response.Write(@"background: url('images/wait01.gif') no-repeat center;");
        this.Response.Write(@"}");
        this.Response.Write(@"</style>");

        this.Response.Write(@"<div id=mydiv class=loading>&nbsp;</div>");
        this.Response.Write(@"<script>mydiv.innerText = '';");
        this.Response.Write(@"</script>");
        this.Response.Write(@"<script language=javascript>;");
        this.Response.Write(@"var dots = 0;");
        this.Response.Write(@"var dotmax = 10;");
        this.Response.Write(@"function ShowWait()");
        this.Response.Write(@"{");
        this.Response.Write(@"var output;");
        this.Response.Write(@"output = 'Wait...';");
        this.Response.Write(@"dots++;");
        this.Response.Write(@"if(dots>=dotmax)dots=1;");
        this.Response.Write(@"for(var x = 0;");
        this.Response.Write(@"x < dots;x++)");
        this.Response.Write(@"{");
        this.Response.Write(@"output += '.';");
        this.Response.Write(@"}");
        this.Response.Write(@"mydiv.innerText =  output;");
        this.Response.Write(@"}");
        this.Response.Write(@"function StartShowWait()");
        this.Response.Write(@"{");
        this.Response.Write(@"mydiv.style.visibility = 'visible'; ");
        this.Response.Write(@"window.setInterval('ShowWait()',1500);");
        this.Response.Write(@"}");
        this.Response.Write(@"function HideWait()");
        this.Response.Write(@"{");
        this.Response.Write(@"mydiv.style.visibility = 'hidden';");
        this.Response.Write(@"window.clearInterval();");
        this.Response.Write(@"}");
        this.Response.Write(@"StartShowWait();");
        this.Response.Write(@"</script>");

        this.Response.Flush();
        Thread.Sleep(500);
    }

    private DataTable GetData(SqlCommand cmd)
    {
        DataTable dt = new DataTable();
        String strConnString = System.Configuration.ConfigurationManager.
             ConnectionStrings["conn"].ConnectionString;
        SqlConnection con = new SqlConnection(strConnString);
        SqlDataAdapter sda = new SqlDataAdapter();
        cmd.CommandType = CommandType.Text;
        cmd.Connection = con;
        try
        {
            con.Open();
            sda.SelectCommand = cmd;
            sda.Fill(dt);
            return dt;
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            con.Close();
            sda.Dispose();
            con.Dispose();
        }
    }

    public override void VerifyRenderingInServerForm(Control control)
    {
    }
}   

Answer 1

Yes, you cannot do this. In your method, PleaseWaityou call Response.Flush. This sends the current buffered output along with the HTTP headers. HTTP headers are always sent before other response data, so you're out of luck.

, PleaseWait, ( ).

, HTTP - "Single request, single response". , HTTP ( , HTTP- , , - 20 , 20 ).