ASP.NET Web Forms: Button Not Disabling Before File Download

June 03, 2025

0

A complete working example in ASP.NET Web Forms (VB.NET) that:

Disables the button immediately using JavaScript.
Downloads a ZIP file.
Avoids Response.End() by using CompleteRequest().
Optionally shows a loading message during the download.

🔧 ASPX Markup (Download.aspx)

<%@ Page Language="VB" AutoEventWireup="true" CodeFile="Download.aspx.vb" Inherits="Download" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Download File</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:Button ID="btnDownload" runat="server"
            Text="Click here to download"
            OnClick="btnDownload_Click"
            OnClientClick="this.disabled=true; document.getElementById('loading').style.display='block';" />
        <div id="loading" style="display:none; margin-top:10px; font-weight:bold;">Preparing download...</div>
    </form>
</body>
</html>

🔧 Code-Behind (Download.aspx.vb)

Imports System.IO
Partial Class Download
    Inherits System.Web.UI.Page
    Protected Sub btnDownload_Click(sender As Object, e As EventArgs) Handles btnDownload.Click
        ' Retrieve docType from query string or use default
        Dim docType As String = Request.QueryString("docType")
        If String.IsNullOrEmpty(docType) Then docType = "1" ' default or fallback value
        ' Download the file
        DownloadZip(CInt(docType))
    End Sub
    Private Sub DownloadZip(docType As Integer)
        ' Simulate a path based on docType (customize as needed)
        Dim zipPath As String = Server.MapPath("~/files/sample.zip") ' Adjust this path
        Dim file As New FileInfo(zipPath)
        If file.Exists Then
            Response.Clear()
            Response.ContentType = "application/zip"
            Response.AddHeader("Content-Disposition", "attachment; filename=" & file.Name)
            Response.AddHeader("Content-Length", file.Length.ToString())
            Response.WriteFile(file.FullName)
            Response.Flush()
            ' Optional: delete after download (if needed)
            ' file.Delete()
            ' Avoid Response.End() — use CompleteRequest()
            HttpContext.Current.ApplicationInstance.CompleteRequest()
        Else
            ' Handle missing file (optional)
            Response.Write("<script>alert('File not found.');</script>")
        End If
    End Sub
End Class

✅ Folder Structure Example

/YourProject
│
├── Download.aspx
├── Download.aspx.vb
├── /files
│   └── sample.zip      <-- Place a test zip file here

✅ Summary of Key Features

✅ Button disables before postback using OnClientClick.
✅ Clean file download with Response.WriteFile().
✅ No UI updates lost because all visual changes are handled client-side.
✅ Safe CompleteRequest() instead of Response.End().

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