Download file to client after form submission

[captainB]

Hi, I'm building a simple online image gallery that allows users to download full size images for free with one condition - they must submit their name and why the chose the image. I want to use a very simple form for this. I would like to send the file back to the user after the information they submitted is logged into a sql server database.

How can I make sure to send the file only after a user submits the form?
And how can I prevent the user from downloading the file directly? Maybe use the transmitfile() after form validation?

Help is greatly appreciated!

[NitinSawant]

just use simple if condition

if user has submitted name
       then download the file
else
        show error

[captainB]

NitinSawant - Thanks for the reply!

I used a rather simple IF logic and it works:
1. user submits form back to server - download.aspx page.
2. download.aspx.cs (code behind) file checks the form fields (here's the IF logic).
3. if form fields are ok, use the Response.Transmitfile() method to download the file to the user.


But here's where it is getting a bit more complicated-
Instead of sending the file back in the response to the form submission, I would like to send back a "your download will begin shortly" page which will send another request to download the file after it is loaded on the browser (javascript timer?) using a http handler, perhaps.

How will the http handler know that the user did in fact submit a form with valid fields for the requested file?

Thanks, I really appreciate you help!

[Frinavale]

I would recommend using an ASPX page to return a "thumbnail" size photo. That way the user won't be able to download the whole image just by viewing your initial page.

Now if the user wants to open the full size image, they provide their information and click the submit button.

The server will validate the user's information, set a session variable indicating that they're permitted to download the file that they've picked.

I would use Ajax here, that way when the user clicks the Submit button the whole page isn't updated when the validation/session-setting-stuff is done.

When the Ajax request returns it'll either display an error message, or execute a bit of JavaScript that will open a new page, linking the user to another ASPX page that will either return the full size image (if the session variable has been set) or it'll return a message stating that they have to successfully fill out the form before they're allowed to download the picture.

To execute the JavaScript upon return will require you to use the AJAX.NET library. You can specify a JavaScript method to execute whenever the Ajax Request is finished.

To do this you'd use the PageRequestManager and supply an "end request event handler method"......

Eg (JavaScript):

var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_endRequest(endRequestEventHandler);
function beginRequestEventHandler()
{ 
  //.....
}

Using the PageRequestManager might not be the best choice here....you could create a Behavior.js file for the page....

There's a bunch of different ways to do this.

But really you should be using ASPX pages to return the images (don't just use a URL to the photo) so that you can add security to your images.

After that, we can take a look at how to allow the user to download the image after they've submitted the forum......

[captainB]

Frinavale - Thank you for such a detailed response!
Let me see if I am implementing your suggestion correctly:
I have a gallery page with thumbnails, and each thumbnail is a link to a page - download.aspx - which shows the select thumbnail and a simple form with name, city, state and comments fields. The form's submit button uses POST to send the form back to the server - the url it uses is download.aspx.cs, which has methods to save the fields to a sql server database.
After the save is successful, I set a session parameter called ["grant_download"] with a value of <filename>.
Now, lets assume the user has one filename in this variable, and would like to download another image. What would be a good way to maintain a list of filenames? I could concatenate to the variable like so: "image01.jpg_image02.jpg_image03.jpg" and slice the string (using the underscore character) into an array. I'd do this with each download request to check if the file is in the array and is permitted to download.

Since I'm using session variables, what would happen when the session ends as a result of a timeout?
One idea I had was to save these variables in the database along with the client's IP, and maybe compare the file requested and IP to a realtime request, which might eliminate the session timeout problem. However clients with dynamic IPs may cause a problem with this logic.

Any ideas?

Thanks!

[Frinavale]

@captainB
I wasn't quite suggesting this.

I was suggesting creating an ASPX page that would not return HTML...instead it would return a "thumbnail" version of the image.

This "ASPX" page would be called something like "Thumbnail.aspx" and would return a very low resolution image of the original image.

This page would have a Response.ContentType="image/jpg". It would read the image into memory and write a low resolution version of the image directly to the output stream.

For example, the following ASPX page will read return an low-resolution thumbnail image when called.

Private _imagePath As String = "~/images/"
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
 
        'Changing the page's content type to display images
        Response.ContentType = "image/jpg"
 
        'Retrieving the name of the image to display 
        '(Yours is stored in Session)
         imageName = Request.QueryString("imgName")
 
         If String.IsNullOrEmpty(imageName) = False Then
                'Retrieving the picture
                imageUrl = String.Format("{0}{1}", _imagePath, imageName)
 
                If IO.File.Exists(Server.MapPath(imageUrl)) Then
                     Dim fullSizeImg As System.Drawing.Image
                     fullSizeImg = System.Drawing.Image.FromFile(Server.MapPath(imageUrl))
                     ShowThumbnail(fullSizeImg )
                     ' cleaning up the full size image
                      fullSizeImg.Dispose()
               End If
         End If
 
End Sub
 
Private Sub ShowThumbnail(ByVal imageToShow As System.Drawing.Image)
 
        Dim imageWidth As Integer = imageToShow.Width
        Dim imageHeight As Integer = imageToShow.Height
 
        Dim newWidth As Integer
        Dim newHeight As Integer
 
 
        'Shrinking the image if it is large than the thumbnail dimensions
        If imageHeight > _maxHeight Or imageWidth > _maxWidth Then
            Dim scaleFactor As Double
            'determine how to scale image: by height or width
            Dim deltaWidth As Integer = imageWidth - _maxWidth
            Dim deltaHeight As Integer = imageHeight - _maxHeight
 
            If deltaHeight > deltaWidth Then
                'difference between the max height and the height of the picture is greater
                'that the difference between the max width and the width of the picture, so scale by height
                scaleFactor = _maxHeight / imageHeight
            Else
                'difference between the max width and the width of the picture is greater
                'that the difference between the max height and the height of the picture, so scale by width
                scaleFactor = _maxWidth / imageWidth
            End If
            newWidth = CType(scaleFactor * imageWidth, Integer)
            newHeight = CType(scaleFactor * imageHeight, Integer)
        End If
 
        'Creating the thumbnail image
        Dim thumbnailImage As System.Drawing.Image
        thumbnailImage = imageToShow.GetThumbnailImage(newWidth, newHeight, New System.Drawing.Image.GetThumbnailImageAbort(Function() False), IntPtr.Zero)
 
        'Writing the thumbnail image directly to the output stream
        thumbnailImage.Save(Response.OutputStream, ImageFormat.Jpeg)
 
        'cleaning up the thumbnail image
        thumbnailImage.Dispose()
End Sub

Now instead of having the image URL as a path to the image, you'd have the Image URL to the path of the Thumbnail.aspx page.

eg:
<asp:Image ImageURL="Thumbnail.aspx?thePicture.jpg" ID="theLowResImage" />



However, if the user was allowed to download the full size image the Thumbnail.aspx page will return the full size image instead of the thumbnail image.

You'd check your 'grant_download' Session variable to see if which version to return to the user.

@captainB
You can store Arrays in session...............

@captainB
Then the user would no longer be allowed to download the full size images.


In this case you'll have to go with your plan B:
@captainB
However, your Plan B is a little over complicated...I'd use Persistent Cookies instead ;)