Application Not Respoding while BackgroundWorker Thread Working...

Question

HI I am using BackgroundWorker in scanning a document using C#.NET. The problem is that while I call the BackgroundWorker.RunWorkerAsync() method application doesn't respond. After finishing the do work it start responding again. As I know BackgroundWorker thread is used to work at background while user can do work in main UI thread... Please make me correct if i am wrong. Here is the BackgroundWorker code. BackgroundWorker backWorker = new BackgroundWorker(); backWorker.WorkerReportsProgress = true; backWorker.DoWork += new DoWorkEventHandler(backWorker_DoWork); backWorker.ProgressChanged += new ProgressChangedEventHandler(backWorker_ProgressChanged); backWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(backWorker_RunWorkerCompleted); progressBar.Visible = true; backWorker.RunWorkerAsync(); } private void backWorker_DoWork(object sender DoWorkEventArgs e) { try { List docs = new List (); string currFilename; // Create a scanner instance (the user can select if more than one) ItemClass scanner = (ItemClass)wiaManager.Create(ref missing); // Show the standard scanning dialog (this is not a required step...) CollectionClass scans = scanner.GetItemsFromUI( WiaFlag.SingleImage WiaIntent.ImageTypeText) as CollectionClass; // If the user clicks Cancel collection is NULL if (scans != null && scans.Count > 0) { // Transfer any scanned pictures to disk ItemClass scan; foreach (object wiaObj in scans) { scan = (ItemClass)Marshal.CreateWrapperOfType(wiaObj typeof(ItemClass)); // create temporary file for image currFilename = Path.GetTempFileName(); // transfer picture to our temporary file scan.Transfer(currFilename false); // Create a Bitmap from the loaded file (Image.FromFile locks the file...) using (FileStream fs = new FileStream(currFilename FileMode.Open FileAccess.Read)) { // KLUDGE: Must wrap the FromStream Image with a new Bitmap. // Otherwise get OutOfMemoryException later when using ColorMatrix on it. docs.Add(new Bitmap(Image.FromStream(fs))); fs.Close(); } // Don't leave junk behind! File.Delete(currFilename); if (docs.Count > 0) OpenImage(docs[0]); } } } catch (Exception ex) { MessageBox.Show(string.Format(Sorry image did not scan. Please try again.
{0} ex.Message) Scanner error); } } private void backWorker_ProgressChanged(object sender ProgressChangedEventArgs e) { progressBar.Value = e.ProgressPercentage; } private void backWorker_RunWorkerCompleted(object sender RunWorkerCompletedEventArgs e) { progressBar.Visible = false; backWorker.ReportProgress(0); } The time where application is not responding to user is while executing the code that I have made bold // transfer picture to our temporary file scan.Transfer(currFilename false); The output is always ok. I just need to make application respond while scanning document at background. Please help me if you can. Thanks in advance.

Khalid Mushtaq · Answer

I have removed the progressbar and backgroundworker reportprogress property but the result is still same..

Khalid Mushtaq · Answer

GetItemsFromUI just shows the standard scanning dialog as commented.. It contains 3 buttons one for preview one for scan and one for cancel... when I press scan button the if condition starts executing if there is any item in the scans and then the GUI thread doesn't respond otherwise if I click on preview or cancel.. GUI thread remains responsive. I hope it will make you understand to find out my problem. Thanks.

Sam Hobbs · Answer

There is a lot in the code that I don't know about. I don't know how the scanner software works or what it is doing. I don't kinow if it can work in a background thread; it might be the problem. The name GetItemsFromUI sounds suspicious; I assume that the form is being accessed from the thread and that could be a problem.

Jure · Answer

I don't really know how BackgroundWorker actually works under the hood but I'd remove ProgressChangedEventHandler completely (if you haven't tried that already). If a background thread makes GUI freeze then it might be updating or invoking the GUI thread too frequently... At least that's the first thing that comes to my mind. I myself use the timer specifically because of control over the frequency of updates. I hope this helps.

Khalid Mushtaq · Answer

Thanks for the reply.. The problem is that if I don't use the progress bar. the situation is same. I am surprised why backgroundworker hangs the GUI for almost 10 seconds at the specific command i.e. scan.Transfer(currFilename false). Thanks again.

Jure · Answer

I hope this is not a total waste of posting since I only skimmed through your code but GUI might be freezing because the progress bar is being updated too often. I do such stuff with ThreadPool then start a timer that updates progress bar. For example: ThreadPool.QueueUserWorkItem(new WaitCallback(yourFunction) ); //  goes for the object the object can't be null The function: private void yourFunction(object o) { count_prog = 0; count_all = // count all items to process this.BeginInvoke(new WaitCallback((object o) => this.timer.Start())); // do your stuff increment count_prog after each item this.BeginInvoke(new WaitCallback((object o) => this.timer.Stop())); } Set the timer to interval of ~100ms but don't go too low. Also you can set the progress bar's boost animation to be more responsive by writing: this.progBar.MarqueeAnimationSpeed = 50; //default is 100 Now for the timer: this.timer.Tick += new System.EventHandler((object o EventArgs e) => { this.progBar.Value = (int)((float)count_prog / count_all * 100); }); If you need to update anything else GUI-wise from the ThreadPool thread use BeginInvoke such as: this.BeginInvoke(new WaitCallback((object o) => { this.statusLabel.Text = (string)o; }) status); //status is the object to pass EDIT: Actually if you use lambda expression for BeginInvoke method you don't have to pass anything because all the stuff that's in scope in the function is also in scope inside the lambda block of code. You have to use BeginInvoke because only the main thread (STAThread) has control over GUI.