Forcing A Postback Asp.Net

Question

Please take a look at the following click event...

 Protected Sub btnDownloadEmpl_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnDownloadEmpl.Click
        Dim emplTable As DataTable = SiteAccess.DownloadEmployee_H()
        Dim d As String = Format(Date.Now, "d")
        Dim ad() As String = d.Split("/")
        Dim fd As String = ad(0) & ad(1)
        Dim fn As String = "E_" & fd & ".csv"
        Response.ContentType = "text/csv"
        Response.AddHeader("Content-Disposition", "attachment; filename=" & fn)
        CreateCSVFile(emplTable, Response.Output)
        Response.Flush()
        Response.End()
        lblEmpl.Visible = True
    End Sub

This code simply exports data from a datatable to a csv file. The problem here is lblEmpl.Visible=true never gets hit because this code doesnt cause a postback to the server. Even if I put the line of code lblEmpl.Visible=true at the top of the click event the line executes fine, but the page is never updated. How can I fix this?

Answer 1

This line:

lblEmpl.Visible = True

Never gets hit because this line:

Response.End()

Throws a ThreadAbortException

I think a cleaner way to handle this is to create a simple HttpHandler component, and 'open' it in a popup window. (The popup window shouldn't actually open. In most cases the browser will realize it's actually a download, and will suppress the tab/window.)

Research the IHttpHandler interface. They're actually quite simple to implement.

Here's a sample handler. Sorry it took awhile, I got called into a meeting:

public class CensusHandler : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        string fileName = String.Format(
            CultureInfo.CurrentUICulture,
            "E_{0:00}{1:00}.csv",
            DateTime.Today.Month,
            DateTime.Today.Day
            );

        context.Response.ContentType = "text/csv";
        context.Response.AddHeader(
            "Content-Disposition", String.Format(null, "attachment; filename={0}", fileName)
            );

        //Dump the CSV content to context.Response

        context.Response.Flush();
    }

    public bool IsReusable { get { return false; } }
}

OK, try adding a javascript onclick event to trigger the download:

<asp:Button ID="Clickety" runat="server" Text="Click Me!" OnClick="Clickety_Click"
    OnClientClick="window.open('Handler.ashx', 'Download');" />

The regular OnClick event will fire your postback code. The javascript onclick (OnClientClick) event will launch the download via the HttpHandler.