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You can't touch a UI control from a background thread. The worker should ONLY generate the thumbnail and return the resulting image. The code on the UI thread should get that image back from the background worker and update the ListView with it.
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"The worker should ONLY generate the thumbnail and return the resulting image."
But how to return the resulting image? Both BackgroundWorker1DoWork and RunWorkerAsync() return void.
What I want to achieve is this: generate a thumbnail, display it in ListView, generate another thumbnail, display it in ListView, and so on. The problem is that ImageList is linked to that ListView. How do I force the ListView to display the thumbnail instead of waiting for all thumbnails to be created?
modified 4-Nov-17 12:07pm.
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Take a look here, if you haven't already: BackgroundWorker Class (System.ComponentModel)[^] And especially at the sample code at the bottom.
I'd suggest you start only one background worker which produces the thumbnails in a loop and after each one you use its method ReportProgress -- but not for reporting the progress-percentage like in the sample but using the other member of ProgressChangedEventArgs[^] (UserState) to deliver the thumbnail bitmap to your UI thread. Which then does basically the same as in LoadListView but only for a single already generated thumbnail.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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"I'd suggest you start only one background worker which produces the thumbnails in a loop and after each one you use its method ReportProgress"
Like this?
void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
for (var i = 0; i < fileList.Count; i++)
{
Bitmap thumbnail = MakeThumbnail(fileList[i], 200, 200);
backgroundWorker1.ReportProgress(i);
}
}
"but not for reporting the progress-percentage like in the sample but using the other member of ProgressChangedEventArgs[^] (UserState) to deliver the thumbnail bitmap to your UI thread."
This is unclear to me. How do I actually use UserState to deliver the thumbnail? That MSDN example is very simplistic (it just shows UserState). This is advanced stuff for me.
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Look at the listed members of the class 
There's this one: BackgroundWorker.ReportProgress Method (Int32, Object) (System.ComponentModel)[^]
As the first argument you could pass i or calculate the progress in percent (or pass 0 if you don't care) and for the second argument you pass thumbnail.
In your ProgressChanged-eventhandler-method then you'll need to cast ProgressChangedEventArgs.UserState to Bitmap.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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OK. So:
backgroundWorker1.ReportProgress(i, thumbnail);
Now the difficult part:
void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
imageList1.Images.Add((Bitmap)e.UserState);
}
Of course it doesn't work. How do I make the ImageList/ListView to accept this new thumbnail?
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In your original LoadListView() you did more stuff than just adding the Bitmap to the ImageList
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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Yes. I added also a caption for each item. I wanted to fix that later. Anyway, I modified the code a little and it works, but unfortunately the ListView flickers a lot (it's not useable like that):
void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
for (var i = 0; i < fileList.Count; i++)
{
Bitmap thumbnail = MakeThumbnail(fileList[i], 200, 200);
backgroundWorker1.ReportProgress(i, thumbnail);
}
}
void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
imageList1.Images.Add((Bitmap)e.UserState);
ListViewItem caption = new ListViewItem(Path.GetFileName(fileList[e.ProgressPercentage]));
caption.ImageIndex = e.ProgressPercentage;
listView1.Items.Add(caption);
}
void LoadListView()
{
listView1.LargeImageList = imageList1;
backgroundWorker1.RunWorkerAsync();
}
Any idea how to improve it?
modified 4-Nov-17 13:48pm.
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The actual entry in a ListView is a ListViewItem that has been added to its Items collection. Just adding a Bitmap to its ImageList doesn't show an entry. The ImageList is just a "repository" of images that can be utilized by the ListViewItems - which is handy if you want to associate the same image with multiple ListViewItems.
edit: Posted before I saw your edited message with code.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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I updated the previous post. Please read it again.
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Take a look here: ListView.BeginUpdate Method (System.Windows.Forms)[^]
You could call that before you start the background worker and call EndUpdate when the background worker is done (another event which you haven't utilized yet I assume).
If you want to see the items being listed while the background worker is still working you would have to "batch" the Bitmaps (in a List<bitmap> or an array for example) before calling ReportProgress and then pass that List/array/batch for UserState. Then use BeginUpdate at the start of BackgroundWorker1ProgressChanged and EndUpdate at the end. And then play around with the batch size to find a value you're comfortable with.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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I declared a global const int batchSize = 10 and a global variable counter.
BackgroundWorker1DoWork sends a new batch of thumbnails every batchSize times or if the i counter already reached the limit (fileList.Count - 1):
void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
List<Bitmap> thumbnailList = new List<Bitmap>();
for (var i = 0; i < fileList.Count; i++)
{
Bitmap thumbnail = MakeThumbnail(fileList[i], 200, 200);
thumbnailList.Add(thumbnail);
if (((thumbnailList.Count == batchSize) && (batchSize < fileList.Count)) ||
(i == (fileList.Count - 1)))
{
backgroundWorker1.ReportProgress(i, thumbnailList);
thumbnailList.Clear();
}
}
}
And the second function:
void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
List<Bitmap> list = (List<Bitmap>)e.UserState;
listView1.BeginUpdate();
if (list.Count > 0)
{
foreach (Bitmap tn in list)
{
imageList1.Images.Add(tn);
ListViewItem caption = new ListViewItem(Path.GetFileName(fileList[counter]));
caption.ImageIndex = counter;
listView1.Items.Add(caption);
counter++;
}
}
listView1.EndUpdate();
}
Unfortunatelly it doesn't work. thumbnailList.Count from the second function reports 0, so apparently thumbnailList.AddRange((List<Bitmap>)e.UserState); is one of the problems. Any ideas?
PS: I uploaded the test project here: TinyUpload.com[^] If you want to test it, drag and drop pictures over the form.
modified 4-Nov-17 16:37pm.
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List<Bitmap> thumbnailList = new List<Bitmap>();
backgroundWorker1.ReportProgress(i, thumbnailList);
Bitmap thumbnail = (Bitmap)e.UserState;
..... = (notbitmapbutsomethingelse)e.UserState; 
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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I updated the previous post. The app still doesn't work. 
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When you pass the list via ProgressChangedEventArgs you pass a reference to the (currently) one and only existing instance of it which you immediately Clear() afterwards (I missed that previously). Instead of clearing it, create a new List<Bitmap> for the next batch.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
at System.ThrowHelper.ThrowArgumentOutOfRangeException()
at System.Collections.Generic.List`1.get_Item(Int32 index)
at test1.MainForm.BackgroundWorker1ProgressChanged(Object sender, ProgressChangedEventArgs e)
at System.ComponentModel.BackgroundWorker.OnProgressChanged(ProgressChangedEventArgs e)
at System.ComponentModel.BackgroundWorker.ProgressReporter(Object arg)
--- End of inner exception stack trace ---
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.UnsafeInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.Delegate.DynamicInvokeImpl(Object[] args)
at System.Delegate.DynamicInvoke(Object[] args)
at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)
at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)
at System.Windows.Forms.Control.InvokeMarshaledCallbacks()
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at test1.Program.Main(String[] args)
Any idea how to fix this bug? I think it's related to those Bitmap lists.
By the way, this is the current code:
void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
List<Bitmap> thumbnailList = new List<Bitmap>();
for (var i = 0; i < fileList.Count; i++)
{
Bitmap thumbnail = MakeThumbnail(fileList[i], thumbnailSize, thumbnailSize);
thumbnailList.Add(thumbnail);
if (((thumbnailList.Count == batchSize) && (batchSize < fileList.Count)) ||
(i == (fileList.Count - 1)))
{
backgroundWorker1.ReportProgress(i, thumbnailList);
thumbnailList = new List<Bitmap>();
}
}
}
void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
List<Bitmap> thumbnailList = (List<Bitmap>)e.UserState;
listView1.BeginUpdate();
if (thumbnailList.Count > 0)
{
foreach (Bitmap thumbnail in thumbnailList)
{
imageList1.Images.Add(thumbnail);
ListViewItem caption = new ListViewItem(Path.GetFileName(fileList[counter]));
caption.ImageIndex = counter;
listView1.Items.Add(caption);
counter++;
}
}
listView1.EndUpdate();
}
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Two minor things first:
This:
&& (batchSize < fileList.Count) ..isn't neccessary because it's covered already by this:
|| (i == (fileList.Count - 1)
And this:
if (thumbnailList.Count > 0) ..isn't neccessary because if the list is empty then the foreach-loop will just do nothing.
Now, to the problem at hand - these lines of the exception stack trace:
at System.Collections.Generic.List`1.get_Item(Int32 index)
at test1.MainForm.BackgroundWorker1ProgressChanged(Object sender, ProgressChangedEventArgs e) ..tell you that the problem occurs when trying to access a List<> with its indexer in BackgroundWorker1ProgressChanged. The only candidate for this is:
fileList[counter] So it seems somehow your counter gets out of sync with the "actual progress". No idea why; maybe it's in the code you haven't shown here.
But that's a point I planned to suggest to you to improve anyway: Change it so that BackgroundWorker1ProgressChanged doesn't need to assume the index of the "delivered" bitmap in fileList by delivering not only the bitmaps but the bitmaps with their index. Either create a class for that (with Bitmap and Index as properties) and store instances of that class in the list (instead of just the Bitmap) or use a Tuple<Bitmap, int> instead of the custom class. Or, instead of using the index, use the filename and ListViewItem.ImageKey[^] instead of ImageIndex. That way you wouldn't need fileList in BackgroundWorker1ProgressChanged at all.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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Thanks for help. I already fixed the minor issues, now I'm trying to improve that index thing.
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Again, thanks a lot for your help.
I created a class Thumbnail:
public class Thumbnail
{
public Bitmap thumbnail;
public string filename;
public int index;
}
I added also a new feature: progress update. The last version of those functions:
void LoadThumbnails()
{
backgroundWorker1.RunWorkerAsync();
}
void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
List<Thumbnail> thumbnailList = new List<Thumbnail>();
for (var i = 0; i < fileList.Count; i++)
{
Thumbnail item = new Thumbnail();
item.thumbnail = MakeThumbnail(fileList[i], thumbnailSize, thumbnailSize);
item.filename = Path.GetFileName(fileList[i]);
item.index = i;
thumbnailList.Add(item);
if ((thumbnailList.Count == batchSize) || (i == (fileList.Count - 1)))
{
backgroundWorker1.ReportProgress(i, thumbnailList);
thumbnailList = new List<Thumbnail>();
}
}
}
void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
List<Thumbnail> thumbnailList = (List<Thumbnail>)e.UserState;
listView1.BeginUpdate();
for (var i = 0; i < thumbnailList.Count; i++)
{
imageList1.Images.Add(thumbnailList[i].thumbnail);
ListViewItem caption = new ListViewItem(thumbnailList[i].filename);
caption.ImageIndex = thumbnailList[i].index;
listView1.Items.Add(caption);
}
listView1.EndUpdate();
if (thumbnailList.Count > 0)
{
Text = "Loading " + thumbnailList[thumbnailList.Count - 1].index + " of " + fileList.Count + " pictures - Thumbnails";
}
}
void BackgroundWorker1RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
{
if (fileList.Count == 1) Text = "1 picture - Thumbnails";
else if (fileList.Count > 1) Text = fileList.Count + " pictures - Thumbnails";
}
modified 5-Nov-17 19:39pm.
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Looks good to me. Is it now working like you want it to?
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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Yes. 
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Cheers
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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The "progress" handler has the same scope as the "worker" method.
"State" is one way to pass a "result" (object, string, whatever; % being used almost never since it implies determinism) to the "progress" handler.
The progress handler (as well as "completed") run on the UI thread.
"Worker" can actually put say 10 images on a cuncurrent queue and then invoke "progress" every 10 to post the queue to the UI; so "state" is only one way to report "progress".
"(I) am amazed to see myself here rather than there ... now rather than then".
― Blaise Pascal
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I'm unsure of the syntax for this..
Assume I have 3 bool methods.. I want to run them in order MethodA(), MethodB(), then MethodC().
As each completes I want to check the return value. If it's true, run the next task. If it's false, stop right there.
Can someone show me how this is done?
Thanks
If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.
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A real simple way is like this:
if (MethodA()){
if (MethodB(){
if (MethodC(){
}
}
}
There are two kinds of people in the world: those who can extrapolate from incomplete data.
There are only 10 types of people in the world, those who understand binary and those who don't.
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