Visual Basic

14-Mar-11 19:50

This is my first time building a background thread. It's working but I would really appreciate an opinion or two as to whether I'm doing it the right way. I'll keep it brief as possible:

The app includes a TreeView and ListView, and when a TreeNode is clicked the ListView displays image files from the corresponding directory. I wanted the thumbnail images in the ListView to retain their aspect ratio so I'm drawing a scaled image inside an empty 64x64 thumbnail. If a dir has a lot of large files this can be quite slow.

So, I wrote a sub that gets images for the ListView via ExtractAssociatedIcon to populate the ImageList in the short term, then afterward creates a background thread (MakeThms) to replace the icons in the ListView's ImageList with thumbnails from the original images. The first sub gets the file names and adds the ListViewItems with system icons for the images:

  Private Sub PopulateListView()
   'MakeThms is the name of the thread
    Try
      If MakeThms.IsAlive Then
        MakeThms.Abort() 'stop thread if another TreeNode is clicked while it's running
      End If
    Catch ex As Exception
     'prevent exception if MakeThms is nothing
    End Try

   'code to populate ListView via ExtractAssociatedIcon - omitted for brevity
   
   'then create the thread to make the scaled thumbnails
    Try
      MakeThms = New Thread(AddressOf BuildThumbnails)
      MakeThms.Start()
    Catch ex As Exception
      MsgBox(ex.ToString, MsgBoxStyle.Exclamation, title)
    End Try
  End Sub


'This is the code used by the background thread...
  Private Sub BuildThumbnails()
    For z As Integer = 0 To images.Count - 1 'images is a string collection of the image file names
      '
      '
      'code to create the scaled thumbnails (thm)
      '
      '
   
      'This is the ImageList used by the ListView (lvFiles)
      'Each image in the ImageList is changed as the new thumbnails are created
      imgList.Images(z) = thm
    Next z
    'redraw the ListView once the thumbnails are created
    lvFiles.Invalidate()
  End Sub

I've been testing it for an hour or so and so far no problems. But I'd appreciate any advice if someone sees a potential problem with how I'm getting it done.

Everybody SHUT UP until I finish my coffee...

Groulien14-Mar-11 21:28

Groulien

14-Mar-11 21:28

I rarely touch VB so I might be wrong, but;
if the BuildThumbnails function fails to convert the first thumbnail, you won't have any thumbnails at all.
(This does not get caught by the try-catch block at MakeThms.Start())

A try-catch block in the BuildThumbnails function would be nice, that way you'll at least have the scaleable thumbnails.

Alan Burkhart15-Mar-11 4:52

15-Mar-11 4:52

nbgangsta wrote:
...if the BuildThumbnails function fails to convert the first thumbnail, you won't have any thumbnails at all.
(This does not get caught by the try-catch block at MakeThms.Start())

True, I need to add a Try Catch block there. I'm looking at it again this morning and thinking maybe I should use a BackGroundWorker instead of creating my own threads. Sometimes the info at MSDN just bounces off my head instead of soaking in. Smile | :)

Everybody SHUT UP until I finish my coffee...

Luc Pattyn15-Mar-11 0:01

15-Mar-11 0:01

I have a number of comments:

1.
I don't like Thread.Abort() as it may leave things in an unknown state. I prefer a cooperative way of terminating a thread, which implies the thread code itself checks an exit condition regularly (requiring some code, and causing some exit delay).

Note: when cooperative, you don't need a new thread each time, it could simply switch to the new job when leaving the old one.

2.
I detest empty catch blocks, I do appreciate you had a comment in there.
If all it does is cope with MakeThms possibly being Nothing, then better test for that explicitly.

3.
lvFiles.Invalidate() is forbidden in anything but the main thread. Don't tell me you are either running a very old .NET version (1.x) or have set lvFiles.CheckForIllegalCrossThreadCalls false? For your penitence read this[^].

4.
when using real Thread instances you should consider setting IsBackground true; if not such running thread could prevent your app from exiting when the main form gets closed.

5.
This situation would be better served by a BackgroundWorker which solves 3 and 4 for you, as it borrows a Thread from the ThreadPool, runs it in the background, and terminates by executing its RunWorkerCompleted handler on the main thread.

6.
when multi-threading, things could go terribly wrong when shared data isn't properly synchronized. I can't tell in your case, just assume the main thread sets imgList or imgList.Images to Nothing while your extra thread is running.

Smile | :)

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Alan Burkhart15-Mar-11 4:48

15-Mar-11 4:48

Luc Pattyn wrote:
1.
I don't like Thread.Abort() as it may leave things in an unknown state. I prefer a cooperative way of terminating a thread, which implies the thread code itself checks an exit condition regularly (requiring some code, and causing some exit delay).

I wasn't sure exactly how to terminate the thread, and wasn't sure if I needed to explicitly end it in this case because it just ran through the "z" loop once and exited the sub. In a case like this, is the thread still alive once the sub has finished?

Luc Pattyn wrote:
2.
I detest empty catch blocks, I do appreciate you had a comment in there.
If all it does is cope with MakeThms possibly being Nothing, then better test for that explicitly.

I should have explained myself better in the comments. I don't usually leave empty catch blocks. I'm still studying in this area. If I do something like If MakeThms.Equals(Nothing) Then I get a null reference exception.

Luc Pattyn wrote:
3.
lvFiles.Invalidate() is forbidden in anything but the main thread. Don't tell me you are either running a very old .NET version (1.x) or have set lvFiles.CheckForIllegalCrossThreadCalls false? For your penitence read this[^].

I'm running .Net 3.5 / VS 2008 Express. I haven't set lvFiles.CheckForIllegalCrossThreadCalls to false. I do get an exception if I use lvFiles.Refresh(). I was pretty sure I shouldn't do this, but amazingly it worked. My primary test directory has 90 images, each 1900x1080 and about 800KB. It takes about 3 seconds to completely rewrite the ImageList. If I quickly select another directory in the TreeView before it's done, it appears to work without any problems. But I was curious since I was touching another thread. I'll dig into the linked article after posting here. Thanks!

Everybody SHUT UP until I finish my coffee...

Luc Pattyn15-Mar-11 4:53

15-Mar-11 4:53

1. A thread that runs out of code just terminates; pretty similar to a method that reaches its end and returns.

2. of course If MakeThms.Equals(Nothing) Then is no good, when MakeThms is Nothing, you can't dereference it, so whatever is following the period will cause a bomb. Just do If MakeThms = Nothing Then.

3. Well I am surprised. If that is allowed, then I'll find some use for it indeed. I'll investigate.

Smile | :)

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Alan Burkhart15-Mar-11 5:19

15-Mar-11 5:19

Luc Pattyn wrote:
1. A thread that runs out of code just terminates; pretty similar to a method that reaches its end and returns.

I was thinking that was the case. Thanks for confirming it. As I told nbgangsta, sometimes the info at MSDN just bounces off my head instead of soaking in. That's when I end up in the CP forums.

Luc Pattyn wrote:

3. Well I am surprised. If that is allowed, then I'll find some use for it indeed. I'll investigate.

MakeThms = Nothing won't work because "=" isn't supported for threads. If you do investigate Invalidate(), I'd enjoy knowing what you find out. I'm going to try out BackgroundWorker instead of creating my own thread. At first blush it seems less than intuitive to use, but probably safer.

Everybody SHUT UP until I finish my coffee...

Luc Pattyn15-Mar-11 5:34

15-Mar-11 5:34

Alan Burkhart wrote:
MakeThms = Nothing

My mistake (and my rusty VB); should be If MakeThms Is Nothing Then (or IsNot). You should have known that, as should I.

Smile | :)

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Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.

Ado,vb .net datagridview datasource runtime

Аslam Iqbal15-Mar-11 8:18

Аslam Iqbal

15-Mar-11 8:18

Good reply.

Member 389234313-Mar-11 23:22

Member 3892343

13-Mar-11 23:22

Hello,
I want to set to a datagrid view it's datasource property at runtime, here is the code which not works maybe needs a refresh.

Me.DataGridView1.DataSource = dataset11

Thank you
Kyriakos

Kyriakos

Re: Ado,vb .net datagridview datasource runtime

Simon_Whale13-Mar-11 23:43

Simon_Whale

13-Mar-11 23:43

Me.DataGridView1.DataSource = dataset11.tables(0)

should work for you

As barmey as a sack of badgers

Dude, if I knew what I was doing in life, I'd be rich, retired, dating a supermodel and laughing at the rest of you from the sidelines.

Re: Ado,vb .net datagridview datasource runtime

Dalek Dave14-Mar-11 0:06

Dalek Dave

14-Mar-11 0:06

That looks good to me! (no pun intended).

------------------------------------

I will never again mention that I was the poster of the One Millionth Lounge Post, nor that it was complete drivel. Dalek Dave

CCC Link[^]
Trolls[^]

Writing to debug log

cstrader23213-Mar-11 14:50

cstrader232

13-Mar-11 14:50

I have some code within try...catch blocks that might not always execute properly. One error that might occur is a System.InvalidCastException error. I trap the error, but can I prevent the error from being written to the log?

Thanks

Re: Writing to debug log

Luc Pattyn13-Mar-11 15:04

Access to the registry key is denied.

13-Mar-11 15:04

And who is writing those exceptions to the log? remove the code that does that. Or, much better, do what it takes not to have those casting exceptions in the first place (can't provide detailed help witout seeing the offensive code).

Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum

Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.

Re: Writing to debug log

Bernhard Hiller14-Mar-11 2:26

Bernhard Hiller

14-Mar-11 2:26

Catch the specific Exceptions first, then the generic Exception:

Try
   'some code
Catch System.InvalidCastException
   'do something
Catch Exception
   'log it and do something
End Try

But of course you should better find the reason for those InvalidCastException, and get rid of them.

TornadoApps10-Mar-11 22:24

TornadoApps

10-Mar-11 22:24

Ok so here is my issue.

In all the years that I've been coding I have never come accross this issue or error. For the past 3 months I have been working on two separate apps. Initialy I started out with Win Vista and things were going great except for how slow my comp was running. So I reinstalled the Vista OS. Everything was going great until Mar 7th when this error just started occuring within my code.

"Access to the registry key 'HKEY_CURRENT_USER\Software\TornadoApps\Swift Budget\User' is denied"

"TornadoApps/Swift" Budget are strings created upon App install or first run of App in VB 2008.

The "Use"r value along with 9 other values were all working fine prior to Mar 7th.

Now this line along with many others like it with defferent strings and values were working just fine till this week. So i figured that I'd just upgrade to Win7 hoping that would fix it. IT didn't. So I reformatted and installed fresh Win7 and still have the error. So I have figured out that it isn't my OS. Cuase it was working in Vista then just quit working.

Can anyone help me.

Dalek Dave10-Mar-11 22:40

Dalek Dave

10-Mar-11 22:40

Sometimes you have to manually set these things.
Obviously I cannot tell you the settings for this particular app, but you will need admin rights.

Perhaps re-installing the app may help too, but remove the regentry before reinstalling.

------------------------------------

I will never again mention that I was the poster of the One Millionth Lounge Post, nor that it was complete drivel. Dalek Dave

CCC Link[^]
Trolls[^]

TornadoApps10-Mar-11 22:51

TornadoApps

10-Mar-11 22:51

Here's the thing.
On Mon both apps, of my design, were working fine.
Both written in Visual Basic 2008.
One I've been working on since Dec the other just started.
Both access the same reg key, "TornadoApps" and sub keys of "Swift Budget" and "ABO".
Both were working on monday and then for no reason jus stopped workin.
Even after I reformated my comp this error still occures.

riced10-Mar-11 22:57

riced

10-Mar-11 22:57

Are you using a network login? If so has the network admin down graded the user id so it is no longer a full admin?
A long shot, but have seen something like this in the past (early days of switching to XP).

Regards
David R
---------------------------------------------------------------
"Every program eventually becomes rococo, and then rubble." - Alan Perlis
The only valid measurement of code quality: WTFs/minute.