Random Function

sal79 · April 21st, 2009, 11:02 AM

I am using the following Random image generator for my background:

Code:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim url As String = RotatePic("/Images/Home/")
        HomePage.Attributes.Add("style", "background:url(" + url + ")")
End Sub
 
Private Function RotatePic(ByVal directoryPath As String) As String
        Dim dirInfo As New DirectoryInfo(Server.MapPath(directoryPath))
        Dim fileList() As FileInfo = dirInfo.GetFiles()
        Dim numberOfFiles As Integer = fileList.Length
        Dim rnd As New Random
        Dim randomFileIndex As Integer = rnd.Next(numberOfFiles)
        Dim imageFileName As String = fileList(randomFileIndex).Name
        Dim fullImageFileName As String = Path.Combine(directoryPath, imageFileName)
        Return fullImageFileName
    End Function

This works most of the time, but sometimes I get no image background at all. How do I ensure that there always is an image?

gbianchi · April 21st, 2009, 11:09 AM

Hi there.. the error is no related with the random function. If the number generated by the fuction is higher than the filelist top, then you would receive an error. Do you see the file name that was generated when the error arrise???

Lee Dumond · April 21st, 2009, 01:33 PM

Pretty easy to see you have two problems here. Good news is that neither of them involves the Random. That part is perfectly fine.

The main problem is that this code will never work if and when there is a space in the image name. This is because you have not placed single-quotes around the url in your Attributes.Add string. So, whenever there is a space in the URL, the browser's CSS interpreter chokes.

Here is the simple fix for that:

Code:

HomePage.Attributes.Add("style", "background:url('" + url + "')")

The second problem, which you may not actually care about, is that this function does not properly resolve the url coming back, so that this code will never work for pages below the root directory. Like I said, if this is for your home page, maybe that's okay with you. However, you can make this more flexible by fixing it so that it resolves correctly no matter where the page is in your folder structure.

Code:

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim url As String = RotatePic("~/Images/Home/")
        HomePage.Attributes.Add("style", "background:url('" + url + "')")
    End Sub
 
    Function RotatePic(ByVal directoryPath As String) As String
        Dim dirInfo As New DirectoryInfo(Server.MapPath(directoryPath))
        Dim fileList() As FileInfo = dirInfo.GetFiles()
        Dim numberOfFiles As Integer = fileList.Length
        Dim rnd As New Random
        Dim randomFileIndex As Integer = rnd.Next(numberOfFiles)
        Dim imageFileName As String = fileList(randomFileIndex).Name
        Dim fullImageFileName As String = Path.Combine(directoryPath, imageFileName)
        Dim fullImageUrl As String = ResolveUrl(fullImageFileName)
      Return fullImageUrl
    End Function

sal79 · April 22nd, 2009, 04:11 PM

that doesn't seem to solve the problem. this is the home page, so the path is fine and all the images are named the same way (homeimage1.jpg, homeimage2.jpg). If I load the page and refresh a couple of times, the images fail to show up at least once. It is not browser specific, it happens in IE, Firefox, and Safari

Lee Dumond · April 22nd, 2009, 04:21 PM

Wow, that is weird. Except for the circumstances I stated, I cannot think of any other reason this could be happening. I have reproduced your example on my end and refreshed a billion times and this never happens.

It would be interesting to see if I could reproduce your issue. Click on my name above and shoot me an email. I will respond, then you'll have my email address. Then, zip up your solution and send it to me, and let me take a look at it.

Lee Dumond · April 27th, 2009, 05:44 PM

Okay... I got your ZIP and figured out your problem.

In the ZIP you sent me, it "appears" as though there are only 5 files in Images/HomePage:

quote2.gif
quote2.gif
quote3.gif
quote4.gif
quote5.gif

However, there are actually 6 files in this directory. (You can confirm this by stepping through the code and inspecting the value of fileList.Length, which is 6.)

If you step through and inspect the value of fileList, you will see the 6th file is the Thumbs.db file.

Thumbs.db is a hidden operating system file, so by default you cannot see it in Windows Explorer. If you disable the option that hides protected operating system files, you will be able to see it right away.

As I mentioned, Thumbs.db is the last file in this directory. So, whenever randomFileIndex = 5, it pulls Thumbs.db from the directory, which of course will not display in the browser.

I confirmed this on the client side by using FireBug to inspect your HomePage div when it came up empty.

Code:

<div id="ctl00_cpMainContent_HomePage" class="Quotes" style="background: transparent url(Images/HomePage/Thumbs.db) repeat scroll 0% 0%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;"> </div>

Solution -- get rid of that thumbs.db file. Problem solved.

P.S. I fixed your solution and sent it back to you.