Download / Upload any file and Display Message using C# in ASP.NET

* To display information using message box in ASP.NET page from server side code.

static public void DisplayAJAXMessage(Control page, string msg)
    {
            string myScript = String.Format("alert('{0}');", msg);
            ScriptManager.RegisterStartupScript(page, page.GetType(), "MyScript", myScript, true);      
    }

e.g. : DisplayAJAXMessage(this, "Hello World");
OUTPUT: 

Fig 1: Output of DisplayAjaxMessage

* Download a file from server using link button:

protected void lnkDownloadFile_Click(object sender, EventArgs e)
    {
        try
        {
            string FileName = string.Empty;
            FileName = "demo.xls"; // Any File Name with extension
            string link = Server.MapPath("<~/Folder Name/>" + FileName); //Optional Folder Name
            FileInfo myfile = new FileInfo(link);
            Response.AddHeader("Content-Disposition", "attachment; filename=" + myfile.Name);
            Response.AddHeader("Content-Length", myfile.Length.ToString());
            Response.ContentType = ReturnExtension(myfile.Extension.ToLower());
            Response.TransmitFile(myfile.FullName);
            Response.Flush();
            Response.End();
        }
        catch (Exception ex)
        {
            DisplayAJAXMessage(this, "File Not Found");
        }
    }

private string ReturnExtension(string fileExtension)
    {
        switch (fileExtension)
        {
            case ".xls":
                // case ".csv":
                return "application/vnd.ms-excel";
            default:
                return "application/octet-stream";
        }
    }

* Upload any file to server using C# in ASP.NET 
(Note: This example shows to upload excel file (.xls) , Just change the extension of file as per requirement to upload any type file.)

  protected void btnSave_Click(object sender, EventArgs e)
    {
        try
        {
            if (UploadFile.HasFile) //FileUpload tool from ToolBox
            {
                string Extension = Path.GetExtension(UploadFile.PostedFile.FileName);
                if (Convert.ToString(Extension).ToLower() != ".xls")
                {
                    lblError.Text = "Please select a file with extension .xls before validation";
                    return;
                }
            }
            else
            {
                lblError.Text = "Please select a file with extension .xls before validation";
                return;
            }
            string uploadFileName = Path.GetFileName(UploadFile.PostedFile.FileName);
            int index = uploadFileName.LastIndexOf('.');
            string SheetName = uploadFileName.Substring(0, index);
            string FileName = Path.GetFileName(UploadFile.PostedFile.FileName);
            string FilePath = Server.MapPath("~//" + FileName); //Optional FolderName
            UploadFile.SaveAs(FilePath);
         }
        catch (Exception ex)
        {
           DisplayAJAXMessage(this,"Error::"+ex);
        }
    }

Happy Coding 

Software Developer, Academic Writing Writer